Predestination

Predestination refers to a group of largely religious concepts regarding free will and the role of God. Specifically, it refers to the Calvinist idea that God has already determined who will be saved. It is one of the "Five Points of Calvinism".

Questions
This idea generally leads to endless (and useless) philosophical discussions about free will and the role of God. Also, it leads to questions about whether God can be considered "just" if some people are preselected for damnation and never given a chance to be saved.

If God has already decided in advance who is going to be saved, then humans can do whatever they want on Earth without affecting whether they will be saved or not. If someone has already been chosen to be saved, then that person can live a life of sin and would still be saved. If someone has already been chosen not to be saved, that person can live a life free of sin and would still not be saved. You can sin as much as you want, and it doesn't affect whether you're saved or not. This totally undermines any religion based on predestination. In fact, why even follow the practices of such a religion? Why bother being a Calvinist? If God already decided you are saved, then you can follow any religion you want, you can even be an atheist(!); you are still saved no matter what you do! If God already decided you are not saved, then you are still not saved even if you choose to be Calvinist; you are still not saved no matter what you do!

However, there are those that would argue that true believers wouldn't even think of sinning, believing that those who do sin and take predestination for granted were never truly saved to begin with.

The Bible
-21 can be interpreted to support predestination:

9:18 Therefore hath he mercy on whom he will have mercy, and whom he will he hardeneth. 9:19 Thou wilt say then unto me, Why doth he yet find fault? For who hath resisted his will? 9:20 Nay but, O man, who art thou that repliest against God? Shall the thing formed say to him that formed it, Why hast thou made me thus? 9:21 Hath not the potter power over the clay, of the same lump to make one vessel unto honour, and another unto dishonour?

On the other hand, -4 goes against it:

For [prayers are] good and acceptable in the sight of God our Saviour; Who will have all men to be saved, and to come unto the knowledge of the truth.

You know it had to happen...
Predestination is not the same as prestidigitation, which is a simulation of magic.