He doesn't offer salvation to everyone.
Let's see what the Bible says about salvation, and who it is for.
New Living Translation
For the grace of God has been revealed, bringing salvation to all people.
English Standard Version
For the grace of God has appeared, bringing salvation for all people,
Berean Study Bible
For the grace of God has appeared, bringing salvation to everyone.
Berean Literal Bible
For the grace of God has appeared, bringing salvation to all men,
King James Bible
For the grace of God that bringeth salvation hath appeared to all men,
New King James Version
For the grace of God that brings salvation has appeared to all men,
New American Standard Bible
For the grace of God has appeared, bringing salvation to all people,
There are many more translations saying the very same thing.
If He did, and then some aren't saved, that would mean those who were, knew something, or did something, the others didn't or couldn't.
Doesn't mean that at all. It means those who are saved responded to the simple gospel message.
1 Cor 1:21 - For since in the wisdom of God the world through its wisdom did not know him,
God was pleased through the foolishness of what was preached
to save those who believe.
That would mean they had worked for their salvation in some way,
You need to leave your calvinistic leanings. Responding to the gospel is NOT A WORK in ANY SENSE.
Rom 4-
4 Now to the one who works, wages are not credited as a gift but as an obligation.
5 However, to the one who does not work but trusts God who justifies the ungodly, their faith is credited as righteousness.
Eph 2-
8 For it is by grace you have been saved, through faith—and this is not from yourselves, it is the gift of God—
9 not by works, so that no one can boast.
These 2 passages clearly show that salvation is by grace and NOT works. Reception of a gift cannot be called a work.
You do know that not everyone will become saved, don't you?
Because some REFUSE to believe.
John 5:40 - yet
you refuse to come to me to have life.
Acts 14:2 - But the
Jews who refused to believe stirred up the other Gentiles and poisoned their minds against the brothers.
Acts 19:9 - But some of them became obstinate;
they refused to believe and publicly maligned the Way. So Paul left them. He took the disciples with him and had discussions daily in the lecture hall of Tyrannus.
These verses PROVE that man has a free will. And they express that will by making a choice; they REFUSE to believe.
This means that believing is a choice. Some who hear the gospel believe and many who hear don't.
For those who aren't and are of your belief system, were they doomed to damnation?
This is a great question!! This is what the Bible says about who will be condemned.
John 3:18 - Whoever believes in him is not condemned, but whoever does not believe stands condemned already because
they have not believed in the name of God’s one and only Son.
2 Thess 2:12 - and so that all will be condemned
who have not believed the truth but have delighted in wickedness.
This shows that all who "have not believed" will be condemned.
So, only those who "have not believed" will be doomed to damnation. All by themselves. Because believing is a choice.