This is such a crucial reality for believers to understand as they cling to Jesus and follow him! I have mentioned this before, and we will come to it again, but it is not enough to say that God promises to save us not only now, but also forever, in the future--because that leaves a lot of time in between! And unless you hold to the loss of rewards view and think that apostasy from Jesus and falling away from faith doesn't mean anything (eternally) significant, then "once saved, always saved" just doesn't cut it. That's why it isn't the fifth point of TULIP--the "perseverance of the saints" is. Namely, all those chosen by God and who belong to His Son will infallibly be protected (through faith!) for a salvation ready to be revealed at the last time.
That's why it is essential to see that God promises not only that He has saved and will save on the last day all those who belong to Him, but also that He will sustain, guard, keep, protect and preserve them until the end so that they do not deny Jesus but stay faithful to him, thus receiving the prize and the inheritance. I have mentioned a number of passages earlier in the summer that I believe teach this (Philippians 1:6, I Thessalonians 5:23-24, Romans 5:9-10, Romans 8:28-39, Jude 24-25, etc.). But I have totally forgotten about I Corinthians 1:8-9. Listen to this promise--and realize it is given to a church that, on the surface, would appear to be the most "unspiritual", "unsanctified" group of Christians we know of in the NT! Which simply means that it is grounded in God's power and faithfulness, and not in our ability or performance or willpower.
"I give thanks to my God always for you because of the grace of God that was given you in Christ Jesus, that in every way you were enriched in him in all speech and all knowledge-even as the testimony about Christ was confirmed among you-so that you are not lacking in any spiritual gift, as you wait for the revealing of our Lord Jesus Christ, who will sustain you to the end, blameless in the day of our Lord Jesus Christ. God is faithful, by whom you were called into the fellowship of his Son, Jesus Christ our Lord."
Notice, lastly, that there are no "ifs" attached to this promise. That doesn't mean perseverance and faith and obedience aren't necessary to be saved--they are. But it does mean that the promise is not based on anything inherent in them. Instead, it is actually perseverance and faith that are based on the promise that God will see us through to the end.