Biblical proof of the rapture at the parousia, capitals added for emphasis
15 For this we say unto you by the word of the Lord, that we which are alive and remain unto the COMING of the Lord shall not prevent them which are asleep.
16 For the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first:
17 Then we which are alive and remain shall be CAUGHT UP together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord.
18 Wherefore comfort one another with these words
Verse 15 tells us this is about the parousia. He writes of those who are alive and remain unto the parousia of the Lord.
Here is verse 15 in Greek:
Τοῦτο γὰρ ὑμῖν λέγομεν ἐν λόγῳ Κυρίου, ὅτι ἡμεῖς οἱ ζῶντες οἱ περιλειπόμενοι εἰς τὴν παρουσίαν τοῦ Κυρίου οὐ μὴ φθάσωμεν τοὺς κοιμηθέντας·
See the word παρουσίαν. parousian