Will God allow sin to get us back?
While a child of God ought not to sin, there is no promise to the Christian that he cannot sin. Instead we read of Christians who did fall back into the ways of sin. "This charge I commit to you, son Timothy, according to the prophecies previously made concerning you, that by them you may wage the good warfare, having faith and a good conscience, which some having rejected, concerning the faith have suffered shipwreck, of whom are Hymenaeus and Alexander, whom I delivered to Satan that they may learn not to blaspheme" (I Timothy 1:18-20). You cannot wreck what you do not possess. Hymenaeus and Alexander had faith, but they shipwrecked it.
Even whole churches fell away from the truth. "Nevertheless I have this against you, that you have left your first love. Remember therefore from where you have fallen; repent and do the first works, or else I will come to you quickly and remove your lampstand from its place -- unless you repent" (Revelation 2:4-5). If the church at Ephesus did not change its way, it would fall away permenantly.
We are warned that leaving the truth would continue to be a problem. "Now the Spirit expressly says that in latter times some will depart from the faith, giving heed to deceiving spirits and doctrines of demons" (I Timothy 4:1).
Hence, there are numerous warnings in the Scriptures to hang onto our salvation:
"Therefore let him who thinks he stands take heed lest he fall" (I Corinthians 10:12).
"And you, who once were alienated and enemies in your mind by wicked works, yet now He has reconciled in the body of His flesh through death, to present you holy, and blameless, and above reproach in His sight-- if indeed you continue in the faith, grounded and steadfast, and are not moved away from the hope of the gospel which you heard, which was preached to every creature under heaven, of which I, Paul, became a minister" (Colossians 1:21-23).
"Therefore we must give the more earnest heed to the things we have heard, lest we drift away" (Hebrews 2:1).
"Beware, brethren, lest there be in any of you an evil heart of unbelief in departing from the living God; but exhort one another daily, while it is called "Today," lest any of you be hardened through the deceitfulness of sin. For we have become partakers of Christ if we hold the beginning of our confidence steadfast to the end" (Hebrews 3:12-14).
"Therefore, since a promise remains of entering His rest, let us fear lest any of you seem to have come short of it" (Hebrews 4:1).
"Let us therefore be diligent to enter that rest, lest anyone fall according to the same example of disobedience" (Hebrews 4:11).