Well, you're right to ask because it's arguable and more like speculation on my part. Sadly, it's my style to act like I speak with authority even when I'm guessing! ;)
So there are several reasons--some just pure logic.
1) God is unlikely to fail to win at least half of what He created for His glory. So at a minimum I would guess God will save at least half of mankind, or perhaps just 1/3 of humanity, if He is replacing the 1/3 of angels he lost to Satan. No promises!
Matt 25.31 “When the Son of Man comes in his glory, and all the angels with him, he will sit on his glorious throne. 32 All the nations will be gathered before him, and he will separate the people one from another as a shepherd separates the sheep from the goats. 33 He will put the sheep on his right and the goats on his left.
Gen 1.28 God blessed them and said to them, “Be fruitful and increase in number; fill the earth and subdue it.
Isa 55.11 so is my word that goes out from my mouth: It will not return to me empty, but will accomplish what I desire and achieve the purpose for which I sent it.
2) Israel was kept even when a very small minority was faithful to God. For example, Elijah thought he and only about 7000 men were faithful to God in his time. 1 Kings 19.18. However, instead of just saving 7000 men God continued to reach out to the whole nation.
Rom 11.25 I do not want you to be ignorant of this mystery, brothers and sisters, so that you may not be conceited: Israel has experienced a hardening in part until the full number of the Gentiles has come in, 26 and in this way all Israel will be saved.
That means not that every Jew or citizen of Israel will be saved, but that the political salvation of Israel will present a whole nation filled with saved individuals, even if not all of them will be saved.
3) There are numerous examples in the Scriptures of God using a believer even after they had walked away, so to speak. Moses, Samson, Peter--all Christians have issues at times. If they can return, then many can be forgiven and at least given a 2nd chance, though they may continue to be as weak as when God initially forgave them. If He forgave them once He will forgive them again. If He ignored their failiure the 1st time, He will ignore their failure the 2nd time.
4) Paul said that Christians can produce different kinds of works, some that are to be destroyed, and some that will resound to God's glory.
1 Corinthians 3:12-13 If anyone builds on this foundation using gold, silver, costly stones, wood, hay or straw, their work will be shown for what it is, because the Day will bring it to light. It will be revealed with fire, and the fire will test the quality of each person’s work.
1 John 2.28 And now, dear children, continue in him, so that when he appears we may be confident and unashamed before him at his coming.
This could mean that some Christians will go to heaven with an immediate sense of shame, though they will be forgiven for their failures. It's debatable.
Ultimately, my purpose in even suggesting this is to display my sense of God's grace, that He never tires in forgiving--70 x 7. And personally, I've known friends who could not overcome their sins and yet believed in God and called upon Him for mercy.
I had an alcoholic friend who had accepted Christ and occasionally tried to beat his habit. But he failed, gave up and probably died drunk and asleep in a house fire while he was on fire watch.
I had another friend who had terrible back pain and died apparently hanging himself on Halloween, whether by accident as a prank or by suicide. The guy managed our sound system for our church and was a good guy, and I believe he will be in heaven.
I believe we will all die with some stains on our record, and some stains will be terrible for some. But God's grace covers all sin when we call upon Him sincerely. And I believe God will save all who cast their trust in Him and believe in His word.
That's what I believe, and I'm sticking with it...
This is why the command in scripture was to think on what was positive, and of a good report, and not to let the things the enemy might be doing cause us to grieve or turn to bitterness. In the end, we win, LoL, and there's nothing the enemy can do about it : )
