The Bible teaches eternal security in many different ways.
The Bible speaks with certainty about the possession of a new life
based solely upon faith in Christ as Savior. John 3: 1-16; 5: 24; 10: 28; 20: 31 2.
The Bible refers to this life as “eternal” which means forever
and implies no interruption. John 10: 28; 11: 25-26 3.
Since salvation by grace essentially means that it is a gift,
then it is an unconditional gift, which does not depend on a person’s works, conduct or condition after salvation. Rom. 3: 24; 4: 5; Eph. 2: 8-9 4.

Bible teaches God’s predestining purpose and initial justification result in eventual glorification without exception for every believer. Rom. 8: 29-30; Eph. 1: 4-5 5.
Bible presents eternal salvation as a legal and binding relationship with God that cannot be separated by anyone (including ourselves) or anything. Rom 8:1, 31-39 Bible presents eternal salvation as an irrevocable filial relationship to the Father by adoption, which results in eternal blessings. John 17: 3; Rom. 8: 15-17; Gal. 3: 26 7.

We are sealed with the Holy Spirit, Who guarantees our glorification.
2 Cor. 1: 22; Eph. 1: 13-14; 4: 30 8.
We are kept secure by the power of both the Father and the Son.
John 10: 28-30; 17: 9-12; Jude 24 9.

Since all of our sins (past, present, future) are forgiven by Jesus Christ and His eternally sufficient sacrifice, there is no sin that can cause us to lose our relationship to Him. Col. 2: 13-14; Heb. 10: 12-14 10.
The intercessory prayers of Jesus Christ and His advocacy when we sin
guarantee that our salvation will be completed eternally.
John 17: 9-12, 24; Heb. 7: 25; 1 John 2: 1 11.

Bible speaks of salvation in the passive voice, which indicates that the causality is not with us, but with God; therefore, it is based upon His work not ours.
Eph. 2: 5, 8; 2 Thess. 2: 10; 1 Tim. 2: 4 12.
The Bible demonstrates by example (Abraham, David, Israel) and by precept that God is faithful to His eternal promises even when we are not.
Ps. 89: 30-37; Rom. 3: 3-4; 4: 16; 2 Tim. 2: 13