1. Christians believe God is infinite, omnipotent, omniscient, and omnibenevolent.
2. Christians believe God has a ‘perfect plan’ for his Creation.
3. HOWEVER, Christians also claim that God endowed mankind with Free Will. Keep this in mind - it will come into play later.
4. Christians believe in a concept called ‘sin’:
Definition of Sin (from allaboutgod.com): doing what is wrong or not doing what is right according to God's rules (1 John 3:4). If God says "Do not lie" and you lie, then you have sinned. If God says "Do not steal" and you steal, then you have sinned. According to God, sin separates you from Him (Isaiah 59:2).
5. However, since God is infinite, omnipotent (all-powerful) and omniscient (all-knowing), he must know in advance who will commit sins, when they will be committed, and for what reasons. He must know the circumstances under which they will be committed and he must know all this BEFORE the sin is committed, by his very definition. Otherwise, he is not God.
6. Accepting that God knows beforehand all the sins that mankind will ever commit, these sins must have already been factored into his Plan. So, how can they be wrong if they are in line with God’s Plan? And furthermore, how can Free Will exist if God has already foreseen all of our thoughts and actions?
6.1. Christians at this point will most likely argue that, because of Free Will, it is possible to go against God’s Divine Plan. They will argue that this is, indeed, the definition of Sin itself.
HOWEVER, by admitting that it is possible for a human being to differ from God’s Plan, they are admitting that either:
6.1.1. God is not omnibenevolent, since an all-loving God would, by definition, not allow his Creation to suffer in eternal torment as a result of ‘Free Will’, something endowed to them by Him in the first place; OR
6.1.2. God is not omniscient, since an all-knowing God would have foreseen all possible outcomes of Free Will and planned accordingly; OR
6.1.3. God does not have a Divine Plan, since the existence of such a plan (one created by an infinite, omniscient, omnibenevolent being) contradicts the existence of Free Will; OR
6.1.4. God does not exist.