Calvin
essentially viewed salvation as a work done completely by God. The
believer really has no part to play in salvation, and rightly so, for this is true salvation in Christ. Yet from this Calvinist perspective, the believer doesn't
even have to be willing to be saved, essentially God will force them to be
saved if it's in his will for them to be saved. And the believer doesn't need to respond in faith, faith is given to the believer. The believer essentially doesn't respond at all. God forces it upon them.
Basically Calvin
believed that everyone is predetermined before they are even born to
heaven or hell. Calvin had a very strong view in regard to the
sovereignty of God. He essentially believed that whatever God decrees is
what will happen. There is no free will involved. There
are several problems with this approach to salvation.
First, if you're asserting that God ordained the fall, and that God ordains some to heaven and some to hell, that makes God the author and originator of evil. That's a rather big problem, when considering the moral character of God.
Secondly, when you remove free will you remove any need to respond to the gospel. Essentially Calvin reduces the human race to robots. How does Calvin explain our own responsibility if we're foreordained to heaven or hell? There would be no basis for personal responsibility for sin.
Thirdly, when you suggest predestination in all things you remove the impotence to evangelism, if people are foreordained to heaven or hell, then there is no reason to evangelize. They will get saved somehow if God wants them to.
Fourthly, when you remove the need for perseverance and simply suggest all will persevere because God will force them to, you neglect the very real reality of falling away. Thus you encourage complacency in the body by rejecting the possibility of apostasy/falling away. Believers then falsely think they can never lose their salvation, promoting spiritual laziness, or even departing the church altogether, given the fact that it can't be lost.
Fifthly, why would the Bible constantly tell us to "choose now whom we will obey" (Joshua 24:14-15) and constantly call us to "obey God in all things" (John 14:15)? If all is foreordained by God, there would be no need for any commands, because God would simply force it to happen.
I know, I'm taking these ideas to their logical conclusions. Most Calvinists don't believe these logical conclusions. Yet they remain logical conclusions that prove the ideology is systemically contradictory. Determinism seems the logical conclusion of God decreeing some to salvation and some to hell.