The doctrine of predestination, in its popular form, says that God has chosen some people to be saved. and these chosen (the “elect”) cannot possibly be lost. The rest of mankind (the “reprobate”), whom God has not predestined to salvation, are irrevocably lost and cannot possibly be saved.
In the Bible, predestination means that God purposed, before the foundation of the world, that all men be saved, and he planned a perfect plan to make this possible. He decided, before ever creating man, what he wanted man to be like. That's predestination.
There is a philosophical question about whether God, in determining his purpose, made it impossible for man to reject it. Among pagans there is a view that no man can reject his destiny. However this is not the Bible view...
God has purposed salvation for all who accept his Son Jesus Christ. God never purposed that anyone reject Christ. However, many people do reject Christ, and therefore reject God's purpose, for God "desires all men to be saved" (1Timothy 2:4,6)
God's purpose must be characterized by justice. If God denied his grace to any man, or forced his grace on any man, then his purpose would show partiality rather than justice.
However, "God is not one to show partiality, but in every nation the one who fears him and does what is right is welcome to him" (Acts 10:34-35). That is God's purpose and justice.
God replied to the children of Israel's accusation that the way of God was unfair. God defended his purpose as follows...
20 The soul who sins, he shall die: the son shall not bear the iniquity of the father, neither shall the father bear the iniquity of the son; the righteousness of the righteous shall be on him, and the wickedness of the wicked shall be on him.
21 But if the wicked turn from all his sins that he has committed, and keep all my statutes, and do that which is lawful and right, he shall surely live, he shall not die. 22 None of his transgressions that he has committed shall be remembered against him: in his righteousness that he has done he shall live.
23 Have I any pleasure in the death of the wicked? says the Lord Yahweh; and not rather that he should return from his way, and live?
24 But when the righteous turns away from his righteousness, and commits iniquity, and does according to all the abominations that the wicked man does, shall he live? None of his righteous deeds that he has done shall be remembered: in his trespass that he has trespassed, and in his sin that he has sinned, in them shall he die.
25 Yet you say, The way of the Lord is not equal. Hear now, house of Israel: Is my way not equal? Aren’t your ways unequal?
26 When the righteous man turns away from his righteousness, and commits iniquity, and dies therein; in his iniquity that he has done shall he die. 27 Again, when the wicked man turns away from his wickedness that he has committed, and does that which is lawful and right, he shall save his soul alive. 28 Because he considers, and turns away from all his transgressions that he has committed, he shall surely live, he shall not die.
29 Yet says the house of Israel, The way of the Lord is not equal. House of Israel, are not my ways equal? Aren’t your ways unequal?
30 Therefore I will judge you, house of Israel, everyone according to his ways, says the Lord Yahweh. Return, and turn yourselves from all your transgressions; so iniquity shall not be your ruin. 31 Cast away from you all your transgressions, in which you have transgressed; and make yourself a new heart and a new spirit: for why will you die, house of Israel? 32 For I have no pleasure in the death of him who dies, says the Lord Yahweh: therefore turn yourselves, and live"
(Ezekiel 18:20-32 WEB).
We are "predestined according to the purpose of him who works all things according to the counsel of his will" (Ephesians 1:11).
There is infinite wisdom in the purpose of God. However, we need to get wisdom ourselves by seeking and accepting God's wisdom and counsel. It is our responsibility to be "not unwise but understanding what the will of the Lord is" (Ephesians 5:11).
God has revealed to us his purpose and plan which predestines us to life eternal. We should not reject or neglect this counsel of God, but cling to it with all our heart that our true destiny might be fulfilled in the will and grace of God.