This doctrine states that God has chosen, apart from human merit, those whom He is pleased to bring to knowledge of Himself. This divine selection was not based upon God looking forward to see who would receive the offer of the gospel. Nor it is based on God looking into the future to see who will reject the Gospel. Both the preterition of the non-elect and the choosing of the elect are based solely on the counsel of God's own will. Some individuals have been chosen for glory, while others have been prepared for destruction (Prov. 16:4; Rom. 9:22). The act of election took place before the foundation of the world. The doctrine of unconditional election does not diminish man's responsibility to believe in the redeeming work of God the Son. Scripture presents a tension between God's sovereignty in salvation and man's responsibility to believe which it does not try to resolve. Divine sovereignty and human responsibility are both true.
I worked so hard to make the above paragraph succint that it hardly seems necessary to say much more. One need only to look through the Scriptures to any passage where God's people are said to be elect or chosen, and when we inquire why they are so elect or chosen, Scripture with unified testimony repeatedly affirms that it is due to nothing more than God's sovereign good pleasure.
Case in point,
For this is what the promise said: “About this time next year I will return, and Sarah shall have a son.” And not only so, but also when Rebekah had conceived children by one man, our forefather Isaac, though 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—she was told, “The older will serve the younger.” As it is written, “Jacob I loved, but Esau I hated.” What shall we say then? Is there injustice on God's part? By no means! For he says to Moses, “I will have mercy on whom I have mercy, and I will have compassion on whom I have compassion.” So then it depends not on human will or exertion, but on God, who has mercy. For the Scripture says to Pharaoh, “For this very purpose I have raised you up, that I might show my power in you, and that my name might be proclaimed in all the earth.” So then he has mercy on whomever he wills, and he hardens whomever he wills. You will say to me then, “Why does he still find fault? For who can resist his will?” But 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?” Has the potter no right over the clay, to make out of the same lump one vessel for honorable use and another for dishonorable use? What if God, desiring to show his wrath and to make known his power, has endured with much patience vessels of wrath prepared for destruction, in order to make known the riches of his glory for vessels of mercy, which he has prepared beforehand for glory—even us whom he has called, not from the Jews only but also from the Gentiles? As indeed he says in Hosea,
“Those who were not my people I will call ‘my people,’
and her who was not beloved I will call ‘beloved.’”
“And in the very place where it was said to them, ‘You are not my people,’
there they will be called ‘sons of the living God.’”
Romans 9:9-26 (ESV)