Classical Christian doctrine states that all people have sinned against God, and therefore are bound to spend eternity in hell. Jesus then provided grace so that some people might repent of their sins, confess that Jesus is Lord, and receive forgiveness for their sins as well as salvation.
One of the distinctive doctrines of the Calvinist TULIP is Irresistible Grace. This is the doctrine that God acts first in the process of salvation. He gives his grace to certain people, and those people are irresistibly compelled to repent and believe, and as a result, receive salvation. Here is the doctrine as described by the Reformed web site Theopedia:
Irresistible Grace (or efficacious grace) is a doctrine in Christian theology particularly associated with Calvinism which teaches that the saving grace of God is effectually applied to those whom he has determined to save (the elect), whereby in God's timing, he overcomes their resistance to the call of the gospel and irresistibly brings them to a saving faith in Christ.
Those who obtain the new birth do so, not because they wanted to obtain it, but because of the sovereign discriminating grace of God. That is, men are overcome by grace, not finally because their consciences were more tender or their faith more tenacious than that of other men. Rather, the willingness and ability to do God's will are evidence of God's own faithfulness to save men from the power and the penalty of sin, and since man is so corrupt that he will not decide and cannot be wooed to follow after God, sovereign efficacious grace is required to convert him. This is done by the regeneration of the Holy Spirit whereby a fallen man who has heard the gospel is made willing and necessarily turns to Christ in God-given faith.The problem with this doctrine is that humans are considered responsible for their decision whether to believe or not. This is still considered a free act. Calvinists will respond by holding to the doctrine of compatibilism, which is the belief that an act can be causally determined and still free.
The traditional problem with compatibilism is that it is vulnerable to manipulation arguments. No compatibilist has been able to come up with a definition of a free act that can distinguish between a free choice and a compelled choice. Behavior that is manipulated by a psychiatrist with the right combination of drugs is generally not considered free behavior.
The problem is worse with Calvinism. On the Calvinist view, not only is behavior both free and determined, but it is also manipulated by God. God's irresistible grace compels cooperation, and yet any choices it produces are still considered free human choices.
Here is the kicker: If God can compel a human choice and it is still considered free, what if another human compels the choice of another human? Is it considered free? Suppose that Jill, a psychiatrist, puts a kleptomania drug into Jack's cup of lemonade. Suppose that this is done without Jack's consent. Suppose also, that Jack is compelled by an irresistible urge to steal someone's car, attempts to do so and is caught. Was Jack's attempt to steal the car a free choice, and is Jack responsible for it? It would seem not, and any court that could prove Jill's actions would absolve Jack of his behavior and hold Jill accountable for Jack's attempt to steal the car.
Suppose that God has elected Jack unto salvation from before the beginning of the world. Suppose that this was done without Jack's prior permission or consent. Suppose that God then irresistibly compels Jack to repent and believe, and Jack is compelled by an irresistible urge to repent and believe. He does so and receives salvation. Was Jack's choice a free choice? It seems not.
And so compatibilism is simply not an option for any Christian.