Yes Calvinists believe before the foundation of the world, God elected them for salvation, but not because of anything good for bad they had done, or anything good in them. Is this not what the scriptures say?

Romans 9:11
11though they were not yet born and had done nothing either good or bad—in order that God’s purpose of election might continue, not because of works but because of him who calls—

And right after that, the famous question is asked, "Well, wait a minute, is God unjust then?"

Romans 9:
14What shall we say then? Is there injustice on God’s part? By no means! 15For he says to Moses, “I will have mercy on whom I have mercy, and I will have compassion on whom I have compassion.” 16So then it depends not on human will or exertion, but on God, who has mercy.

And then the other famous question, "Well how could God find fault then if salvation is not left to free will, because who can resist his will?"


Romans 9:
19You will say to me then, “Why does he still find fault? For who can resist his will?” 20But who are you, O man, to answer back to God? Will what is molded say to its molder, “Why have you made me like this?” 21Has the potter no right over the clay, to make out of the same lump one vessel for honorable use and another for dishonorable use?

The scriptures say Jesus died for the sheep.
Many of the passages that say "world" etc. were meant to let the Israelites know salvation was not exclusive to them, but for the Gentiles too. This has been a huge misunderstanding in the church to think that world means every single person and there are plenty of scriptures to refute that argument.

Revelation 5:9 tells us what is meant by "world", that Christ died for people of every tribe, language, and nation, not for every single person:

9 And they sang a new song, saying,

“Worthy are you to take the scroll
and to open its seals,
for you were slain, and by your blood you ransomed people for God from every tribe and language and people and nation.

He died for the sheep. The bible divides people into two categories, sheep and goats. And it says Christ died for the sheep. It doesn't say he died for the goats. And the sheep were elected not because of anything good or bad they had done (wouldn't a good thing be to choose Christ if we could, wouldn't that be the very best thing we could do, but it says he chose not based on anything good or bad we had done).

John 10:11
11 I am the good shepherd. The good shepherd lays down his life for the sheep.

You said, "Jesus promised He would give me undeserved favor because I'm a believer."

But I ask, what is in you that made you a believer over someone else?