Calvinism is a branch of Christian doctrine that teaches before the world was God picked and chose those who would be saved, not because of their own goodness but according to His own purpose, and the rest are left to perish.
There are A LOT of verses to back this. But I will only quote a few, as I don't want to drown this post in verses.
What then? Israel has not obtained what it seeks; but the elect have obtained it, and the rest were blinded. Just as it is written: “God has given them a spirit of stupor, Eyes that they should not see And ears that they should not hear, To this very day.” Romans 11
And not only this, but when Rebecca also had conceived by one man, even by our father Isaac (for the children not yet being born, nor having done any good or evil, that the purpose of God according to election might stand, not of works but of Him who calls), it was said to her, “The older shall serve the younger.” As it is written, “Jacob I have loved, but Esau I have hated.”
What shall we say then? Is there unrighteousness with God? Certainly not! For He says to Moses, “I will have mercy on whomever I will have mercy, and I will have compassion on whomever I will have compassion.” So then it is not of him who wills, nor of him who runs, but of God who shows mercy. For the Scripture says to the Pharaoh, “For this very purpose I have raised you up, that I may show My power in you, and that My name may be declared in all the earth.” Therefore He has mercy on whom He wills, and whom He wills He hardens.
You will say to me then, “Why does He still find fault? For who has resisted His will?” But indeed, O man, who are you to reply against God? Will the thing formed say to him who formed it, “Why have you made me like this?” Does not the potter have power over the clay, from the same lump to make one vessel for honor and another for dishonor?
What if God, wanting to show His wrath and to make His power known, endured with much longsuffering the vessels of wrath prepared for destruction, and that He might make known the riches of His glory on the vessels of mercy, which He had prepared beforehand for glory, even us whom He called, not of the Jews only, but also of the Gentiles? Romans 9
And one more:
...and as many as had been appointed to eternal life believed. Acts 13:48
And we know not all in this text believed.
Now, how can God pick and choose and be just? It's pretty simple if you understand law.
If a man kills and is put in prison, he is rightly paying the penalty for his crime, and the judge is not obligated to free him at any time, but the judge, if he is in complete control, can have mercy on him and free him if he so wishes, but again, he is by no means obligated to.
In the same way, men have sinned according their will, no one forcing them to, and are rightly and justly, by law, under condemnation. Is God now obligated to save and forgive those justly under condemnation? No court room would say He is. But He may have mercy on whom HE wills, according to His purpose.
Now, some Christians will say, "But that's unjust!" But how is it? Show me how God is obligated to save any. He was not obligated to save angels that sinned and offered them no chance at repentance, so why is He obligated to save sinners who are justly and rightly under condemnation?
All are under sin and God doesn't have to save any, as He didn't save the angels, BUT He can have mercy on those He wishes. And for what reason would He have mercy on wicked sinners? To the glory of His mercy. In this, you see that man has no room to boast before God, but only to be completely and utterly grateful.
Now, you run into the problem of will. Does God force His elect to come? No. He gives them eyes to see and ears to hear and the ability to come to Him, but the rest He leaves to perish, as He is by no law obligated to save them. He sends His call throughout all the Earth, and those who are His will come, but those who are not will by no means come.
Jesus said, "The good shepherd gives His life for the sheep." Then He says, "But you do not believe because you are not My sheep." Jesus also says, "All the Father has given Me WILL come to Me. And no one will take them out of My hand." What about those that don't come? It's simple. They were not given to Him by the Father. Again, Jesus said, "No man comes unless the Father draw Him."
All sin according to their will and are justly, by law, under the penlaty of sin, and God, by law, is not obligated to save any.
I could explain further, but I'll leave room for people to question.