In 1 John 4:2b, what does John mean by ἐν σαρκὶ ἐληλυθότα? “In flesh [and blood]” or “among humans/men”? And why is this such an important issue?

1Jn 4:2 By this you know the Spirit of God: every spirit that
confesses that Jesus Christ has come in the flesh is from God,

Westcott and Hort / [NA27 variants] Ἐν τούτῳ γινώσκετε τὸ πνεῦμα τοῦ
θεοῦ· πᾶν πνεῦμα ὃ ὁμολογεῖ Ἰησοῦν Χριστὸν ἐν σαρκὶ ἐληλυθότα ἐκ τοῦ
θεοῦ ἐστίν,

See also http://biblehub.com/text/1_john/4-2.htm

Please provide primary source(s) for any historical references.

By the way, my own view is that “John” wrote his letter to correct various misrepresentations of Jesus, such as saying that he was divine or an angel (the message of the antichrists) which he considered idolatry.

Here is the semantic domain of σάρξ, σαρκός from BDAG:

σάρξ, σαρκός, ἡ (Hom.+; ‘flesh’). ① the material that covers the bones
of a human or animal body, flesh lit. 1 Cor 15:39abcd; Hv 3, 10, 4; 3,
12, 1. The pl. (which denotes flesh in the mass [Lucian, Dial. Mort.
10, 5], whereas the sing. rather denotes the substance.—Herodas 4, 61;
Gen 40:19; 1 Km 17:44; 4 Km 9:36; PsSol 4:19; TestJob 13:5; Philo;
Jos., Ant. 12, 211; Just., A I, 26, 7; Mel., P. 52, 383; Ath. 34, 2)
Lk 24:39 v.l.; Rv 19:18, 21 (4 [6] Esdr [POxy 1010, 16] cannibalism
out of hunger, sim. Mel., P. 52, 383; Quint. Smyrn. 11, 245: the
σάρκες of the slain are food for the birds) B 10:4; metaph. Rv 17:16.
It decays 1 Cl 25:3; cp. Ac 2:31 (cp. 2a below). Normally gives forth
an evil odor when burned MPol 15:2. W. bones (s. ὀστέον) 1 Cl 6:3 (Gen
2:23); Lk 24:39; Eph 5:30 v.l. (metaph.). Paul speaks of his illness
as a σκόλοψ τῇ σαρκί (s. σκόλοψ) 2 Cor 12:7. ἡ ἐν σαρκὶ περιτομή the
physical circumcision (cp. Just., D. 10, 1 al.) Ro 2:28; cp. Eph
2:11b; Col 2:13 (ἀκροβυστία 2); Gal 6:13 (ἡ σάρξ=the flesh that is
circumcised); B 9:4. Metaph.: the corrosion on the precious metals of
the rich φάγεται τὰς σάρκας ὑμῶν ὡς πῦρ Js 5:3.—Ign. describes the
elements of the Eucharist as σὰρξ (or αἷμα) Ἰησοῦ Χριστοῦ IRo 7:3;
IPhld 4; ISm 7:1. Also J 6:51–56 urges that one must eat the flesh
(and drink the blood) of the Human One or Son of Man (Just., A I, 66,
2; s. TPhilips, Die Verheissung der hl. Eucharistie nach Joh. 1922;
Bultmann ad loc.; AWikenhauser ’48, 105f).—His anti-Docetic position
also leads Ign. to use the concept ‘flesh (and blood) of p 915
Christ’ in other contexts as well ITr 8:1; IPhld 5:1.—For Mt 16:17;
Gal 1:16; Eph 6:12; and 1 Cor 15:50 s. 3a. ② the physical body as
functioning entity, body, physical body ⓐ as substance and living
entity (Aeschyl., Sept. 622: opp. νοῦς; Ex 30:32; 4 Km 6:30; TestAbr A
20 p. 103, 6 [Stone p. 54] πάντα τὰ μέλη τῆς σαρκός μου; w. καρδία or
ψυχή Alex. Aphr., An. p. 98, 7–10 Br.; Ps 37:8; 62:2; Eccl 2:3; Ezk
11:19; 44:7 a1.; Jos., Bell. 6, 47, Ant. 19, 325; Ar.15, 7) οὔτε ἡ
σὰρξ αὐτοῦ εἶδεν διαφθοράν Ac 2:31 (but s. 1). W. ψυχή 1 Cl 49:6 (Tat.
13:2 al.). W. καρδία Ac 2:26 (Ps 15:9).—Eph 5:29. ἑόρακαν τὸ πρόσωπόν
μου ἐν σαρκί they have seen me face to face Col 2:1. ἕως ἂν τὸν
χριστὸν ἐν σαρκὶ ἴδῃ before he had seen the Messiah in person GJs 24:4
(cp. Lk 2:26). Opp. πνεῦμα (Ath. 31:3; PGM 5, 460 ἐπικαλοῦμαί σε τὸν
κτίσαντα πᾶσαν σάρκα κ. πᾶν πνεῦμα) 1 Cor 5:5; 2 Cor 7:1; Col 2:5; 1
Pt 4:6; Hm 3:1; 10, 2, 6; cp. AcPl Ant 13:17 (=Aa, I 237, 2; s. οἶδα);
also in relation to Christ (though this is disputed) J 6:63; Hs 5, 6,
5–7; cp. 1 Ti 3:16.—ἀσθένεια τῆς σαρκός bodily ailment Gal 4:13; s.
vs. 14. ἀσθενὴς τῇ σαρκί weak in the body Hs 9, 1, 2. ὁ ἀλγῶν σάρκα
the one who is ill in body B 8:6. πάσχειν σαρκί 1 Pt 4:1b. Cp. 2 Cor
7:5. ἡ τῆς σαρκὸς καθαρότης the purity of the body Hb 9:13 (opp.
καθαρίζειν τὴν συνείδησιν vs. 14). σαρκὸς ἀπόθεσις ῥύπου 1 Pt 3:21 (s.
ῥύπος 1). The σάρξ is raised fr. the dead (s. ParJer 6:9; Theoph. Ant.
1, 7 [74, 2]) 1 Cl 26:3; 2 Cl 9:1. ἀνάστασις σαρκός AcPlCor 1:12; 2:24
(σαρκὸς ἀνάστασιν Just., D. 80, 5); cp. ἀναστήσεσθε ἔχοντες ὑγιῆ τὴν
σάρκα AcPlCor 2:32. Of the body of Christ during his earthly ministry
Eph 2:14 (JHart, The Enmity in His Flesh: Exp. 6th ser., 3, 1901,
135–41); Hb 10:20; 1 Pt 3:18; 4:1a; 1J 4:2; 2J 7; B 5:1, 10f; 6:7, 9;
7:5; 12:10; IEph 7:2; Pol 7:1; AcPlCor 2:6b. Married couples form μία
σάρξ (Gen 2:24; s. Ath. 33, 2 τὴν σάρκα πρὸς σάρκα …
κοινωνίαν.—GAicher, Mann u. Weib ein Fleisch: BZ 5, 1907, 159–65) Mt
19:5f; Mk 10:8ab; 1 Cor 6:16; Eph 5:31 (on these passages, TBurkill,
ZNW 62, ’71, 115–20). δικαιώματα σαρκός behind ‘all sorts of
ceremonial washings’ there are regulations that concern the physical
body Hb 9:10.—On ὑποτάγητε τῷ ἐπισκόπῳ ὡς ὁ Χριστὸς τῷ πατρὶ κατὰ
σάρκα IMg 13:2 s. Hdb. ad loc. and MRackl, Die Christologie des hl.
Ignatius v. Ant. 1914, 228.—πνεῦμα δυνάμεως … ὁ θεὸς … κατέπεμψεν εἰς
σάρκα τουτέστιν εἰς τὴν Μαρίαν God sent a powerful spirit (prob. a
ref. to the kind of divine breath that brought the first human being
to life [Gen 2:7]) into flesh, that is, into Mary AcPl Ha 8, 26=BMM
recto 34; s. AcPlCor 1:14. ⓑ as someth. with physical limitations,
life here on earth (ApcEsdr 4:4 p. 28, 3 Tdf. σάρκα ἀνθρωπίνην φορῶ)
θλῖψιν τῇ σαρκὶ ἕξουσιν 1 Cor 7:28. Cp. 2 Cor 4:11; Col 1:24. Of
Christ τὸ σῶμα τῆς σαρκὸς αὐτοῦ his body with its physical limitations
Col 1:22; cp. 2:11 and s. cα below (cp. En 102:5 τὸ σῶμα τῆς σαρκὸς
ὑμῶν; 1QpHab 9:2; Orig., C. Cels. 6, 29, 25).—Of human life: ἀποδημεῖν
τῆς σαρκός MPol 2:2 (s. ἀποδημέω). ἐπιμένειν ἐν τῇ σαρκί Phil 1:24.
ζῆν ἐν σαρκί vs. 22; Gal 2:20. ἐν σ‌. περιπατεῖν 2 Cor 10:3a. ἐν σ‌.
τυγχάνειν Dg 5:8a. ὄντος ἔτι ἐν σ‌. σου AcPlCor 1:6. τὸν ἐπίλοιπον ἐν
σ‌. χρόνον 1 Pt 4:2. ἡ ἐπιδημία τῆς σαρκὸς ταύτης our sojourn in life
2 Cl 5:5. ἐν τῇ σαρκί in our earthly life 8:2. ⓒ as instrument of
various actions or expressions. α. In Paul’s thought esp., all parts
of the body constitute a totality known as σ‌. or flesh, which is
dominated by sin to such a degree that wherever flesh is, all forms of
sin are likew. present, and no good thing can live in the σάρξ Ro 7:18
(cp. Philo, Gig. 29 αἴτιον δὲ τῆς ἀνεπιστημοσύνης μέγιστον ἡ σὰρξ καὶ
ἡ πρὸς σάρκα οἰκείωσις; Sextus 317 ἀγαθὸν ἐν σαρκὶ μὴ ἐπιζήτει. The OT
lays no stress on a necessary relationship betw. flesh as a substance,
and sin. But for Epicurus the σάρξ is the bearer of sinful feelings
and desires as well as the means of sensual enjoyment: Ep. in Plut.,
Mor. 135c; 1087bf; 1089e; 1096c αἱ τῆς σαρκὸς ἐπιθυμίαι. Also Diog. L.
10, 145. Likew. Plut. himself: Mor. 101b ταῖς τῆς σαρκὸς ἡδοναῖς;
672e; 688d; 734a; Ps.-Plut., Mor. 107f σαρκὶ καὶ τοῖς πάθεσι ταύτης;
Maximus Tyr. 33, 7a. Cp. 4 Macc 7:18 τὰ τῆς σαρκὸς πάθη; Philo, Deus
Imm. 143 σαρκὸς ἡδονή, Gig. 29; TestJud 19:4; TestZeb 9:7; ApcMos 25
[p. 14, 2 Tdf.] εἰς τὴν ἁμαρτίαν τῆς σαρκός); Ro 6:19; 7:25 (opp.
νοῦς); 8:3a, 4–9 (cp. Persius 2, 63 scelerata pulpa, which
contaminates devotion to deity), 12f; Gal 5:13, 24; Col 2:23; Jd 23;
AcPlCor 2:11, 15; Dg 6:5 (opp. ψυχή, as Plut., Mor. 101b). Opp. τὸ
πνεῦμα Ro 8:4, 5, 6, 9, 13; Gal 3:3; 5:16, 17ab; 6:8ab; J 3:6; B 10:9.
τὸ μὲν πνεῦμα πρόθυμον, ἡ δὲ σὰρξ ἀσθενής (cp. Orig., C. Cels. 2, 25,
8) Mt 26:41; Mk 14:38; Pol 7:2. σὰρξ ἁμαρτίας sinful flesh Ro 8:3b.
ἐπιθυμία (τῆς) σαρκός (cp. Maximus Tyr. 20, 9f σαρκῶν … ἐπιθυμίας) Gal
5:16; 1J 2:16; B 10:9. Pl. Eph 2:3a, cp. b; 2 Pt 2:18; cp. Ro 13:14.
τὰ ἔργα τῆς σαρκός Gal 5:19 (s. Vögtle at πλεονεξία). τὰ θελήματα τῆς
σαρκός Eph 2:3b. ὁ νοῦς τῆς σαρκός Col 2:18. τὸ σῶμα τῆς σαρκός the
body of (sinful) flesh 2:11; cp. 1:22 and s. b above (cp. Sir 23:17
σῶμα σαρκὸς αὐτοῦ; En 102:5 τῷ σώματι τῆς σαρκὸς ὑμῶν). τὰ τῆς σαρκός
what pertains to (sinful) flesh Ro 8:5b. ἐν (τῇ) σαρκὶ εἶναι be in an
unregenerate (and sinful) state Ro 7:5; 8:8f. τὰ ἔθνη ἐν σαρκί Eph
2:11a. κατὰ σάρκα εἶναι Ro 8:5a; ζῆν vs. 12b; 13; Dg 5:8b; περιπατεῖν
Ro 8:4; 2 Cor 10:2; βουλεύεσθαι 1:17; στρατεύεσθαι 10:3b; cp. IRo 8:3
(opp. κατὰ γνώμην θεοῦ). β. source of the sexual urge. The σάρξ is the
source of the sexual urge, without any suggestion of sinfulness
connected w. it ἐκ θελήματος σαρκὸς ἐγεννήθησαν J 1:13. ⓓ as someth.
attractive 2 Pt 2:10 (a Hebraism, cp. Judg 2:12; 3 Km 11:10; Sir
46:10). S. also 3b. ③ one who is or becomes a physical being, living
being with flesh ⓐ of humans person, human being: πᾶσα σάρξ every
person, everyone (LXX; TestAbr B 7 p. 112, 3 [Stone p. 72]; GrBar
4:10; ApcEsdr 7:7; ApcMos 13 [p. 7, 1 Tdf.]; Mel., P. 55, 400: for
כָּל-בָּשָׂר; s. πᾶς 1aα) Lk 3:6 (Is 40:5); J 17:2; Ac 2:17 (Jo 3:1);
1 Pt 1:24 (Is 40:6); 1 Cl 59:3; 64; 2 Cl 7:6; 17:5 (the last two Is
66:24); AcPlCor 2:6a. οὐ πᾶσα σάρξ no person, nobody (En 14:21
end.—W-S. §26, 10a; B-D-F §275, 4; 302, 1; Rob. 752) Mt 24:22; Mk
13:20; Ro 3:20 (cp. Ps 142:2 πᾶς ζῶν); 1 Cor 1:29 (μή); Gal
2:16.—Though σ‌. in the foll. passages refers to body in its physical
aspect, it cannot be divorced from its conjunction with αἷμα, and the
unit σὰρξ καὶ αἷμα (cp. Sir 17:31; TestAbr B 13 p. 117, 26 [Stone p.
82]; Philo, Quis Div. Rer. Her. 57; Just., D. 135, 6) refers to a
human being in contrast to God and other transcendent beings Mt 16:17;
Gal 1:16; Eph 6:12 (here vice versa, αἷ. καὶ σ‌.). τὰ παιδία
κεκοινώνηκεν αἵματος καὶ σαρκός the children share mortal nature Hb
2:14, but with suggestion of its frailty, as indicated by the context
with its ref. to death. Because they are the opposites of the divine
nature σὰρξ καὶ αἷμα βασιλείαν θεοῦ κληρονομῆσαι οὐ δύναται 1 Cor
15:50 (JJeremias, NTS 2, ’56, 151–59). For Jd 7 s. b next. Cp. AcPl
Ant 13, 17 (=Aa I 237, 2) σαρκί personally (s. οἶδα 2). ⓑ of
transcendent entities ὁ λόγος σὰρξ ἐγένετο J 1:14 (RSeeberg, Festgabe
AvHarnack dargebracht 1921, 263–81.—Artem. 2, 35 p. 132, 27 ἐὰν
σάρκινοι οἱ θεοὶ φαίνωνται; Synes., Dio 6 p. 45b).—Of flesh other than
human: ὀπίσω σαρκὸς ἑτέρας after another kind of flesh (cp. Judg 2:12
ὀπίσω θεῶν ἑτέρων) i.e. of divine messengers who take on σ‌. when they
appear to humans (so Windisch et al.; difft. Frame et al. of same-sex
activity) Jd 7. p 916

…④ human/ancestral connection, human/mortal nature, earthly
descent (Did., Gen. 144, 25) Ἀβραὰμ τὸν προπάτορα ἡμῶν κατὰ σάρκα Ro
4:1 (Just., D. 43, 7 al.). οἱ συγγενεῖς μου κατὰ σάρκα 9:3. τοὺς τῆς
σαρκὸς ἡμῶν πατέρας Hb 12:9. τὸν Ἰσραὴλ κατὰ σάρκα the earthly Israel
1 Cor 10:18 (opp. τὸν Ἰσραὴλ τοῦ θεοῦ Gal 6:16). Of natural descent
τὰ τέκνα τῆς σαρκός children by natural descent Ro 9:8 (opp. τὰ τέκνα
τῆς ἐπαγγελίας). ὁ μὲν ἐκ τῆς παιδίσκης κατὰ σάρκα γεγέννηται Gal
4:23; cp. vs. 29. μου τὴν σάρκα my compatriots Ro 11:14 (s. Gen
37:27).—Of Christ’s physical nature Ro 8:3c; Hb 5:7. Christ is
descended fr. the patriarchs and fr. David (τὸ) κατὰ σάρκα according
to the human side of his nature, as far as his physical descent is
concerned Ro 1:3 (JDunn, Jesus: Flesh and Spirit [Ro 1:3f], JTS 24,
’73, 40–68); 9:5; 1 Cl 32:2; IEph 20:2. The context of 2 Cor 11:18
includes ancestry as a reason for boasting, but σ‌. in this pass.
applies as well to other aspects of Paul’s career and therefore
belongs more properly in 5…

⑤ the outward side of life as determined by normal perspectives or
standards, a transf. sense of 1 and 2. Usually w. κατά indicating norm
or standard σοφοὶ κατὰ σάρκα wise (people) according to human
standards 1 Cor 1:26. καυχᾶσθαι κατὰ (τὴν) σάρκα boast of one’s
outward circumstances, i.e. descent, manner of life, etc. (cp. 11:22)
2 Cor 11:18. κατὰ σάρκα Χριστόν Christ (the Messiah) from a human
point of view or as far as externals are concerned 5:16b, cp. a (κατά
B5bβ and 7a; also VWeber, BZ 2, 1904, 178–88; HWindisch, exc. ad loc.;
Rtzst., Mysterienrel.3, 374–76; FPorter, Does Paul Claim to Have Known
the Historical Jesus [2 Cor 5:16]?: JBL 47, 1928, 257–75; RMoxon, CQR
108, 1929, 320–28). οἱ κατὰ σάρκα κύριοι those who, according to human
standards, are masters Eph 6:5; Col 3:22. ὑμεῖς κατὰ τὴν σ‌. κρίνετε
you judge by outward things, by externals J 8:15. Of the route taken
in one’s earthly life ἡ ὁδὸς ἡ κατὰ σάρκα IRo 9:3.—ἐν σαρκὶ πεποιθέναι
place one’s trust in earthly things or physical advantages Phil 3:3f.
εὐπροσωπῆσαι ἐν σαρκί Gal 6:12. Onesimus is a beloved brother to
Philemon καὶ ἐν σαρκὶ καὶ ἐν κυρίῳ both as a human being (=personally,
in the external relationship betw. master and slave) and as a
Christian Phlm 16. ὑμῶν δὲ ἐν σαρκὶ ἐπισκόπῳ IEph 1:3 (cp. IMg
3:2).—HWindisch, Taufe u. Sünde 1908; EBurton, ICC Gal. 1920, 492–95;
WSchauf, Sarx 1924; WBieder, Auferstehung des Fleisches od. des
Leibes?: TZ 1, ’45, 105–20. W. special ref. to Paul: Ltzm., Hdb. exc.
on Ro 7:14 and 8:11; Lohmeyer (ἁμαρτία 3a); EKäsemann, Leib u. Leib
Christi ’33; RGrant, ATR 22, ’40, 199–203; RBultmann, Theologie des
NTs ’48, 228–49 (Engl. tr. by KGrobel, ’51 I, 227–59); LMarshall,
Challenge of NT Ethics ’47, 267–70; E Schweizer, Die hellenist.
Komponente im NT sarx-Begriff: ZNW 48, ’57, 237–53; two in KStendahl,
The Scrolls and the NT, ’57: KKuhn, 94–113 and WDavies, 157–82;
JPryke, ‘Spirit’ and ‘Flesh’ in Qumran and NT: RevQ 5, ’65, 346–60;
DLys, La chair dans l’AT ’67; ASand, D. Begriff ‘Fleisch’ ’67 (Paul);
RJewett, Paul’s Anthropological Terms ’71, 49–166. On Ign.:
CRichardson, The Christianity of Ign. of Ant. ’35, esp. 49 and 61. S.
also the lit. s.v. πνεῦμα, end.—B. 202. DELG. M-M. EDNT. TW. Spicq.
Sv.

Arndt, W., Danker, F. W., & Bauer, W. (2000). A Greek-English lexicon
of the New Testament and other early Christian literature (3rd ed., p.
916). Chicago: University of Chicago Press.