Someone who offers Pascals Wager is arguing that to believe in God is a better bet than not believing in God. If you believe and God exists, youll go to heaven and avoid hell; if you believe and are wrong, you lose nothing. If you dont believe in God and God does exist, youll lose heaven and go to hell; if youre right, then you gain nothing. There are a lot of problems with this argument.
The first problem lies in the implicit yet unstated assumption that we already know which god we should believe in. That assumption, however, is not necessary to the argument, and thus the argument itself does not explain which religion a person should follow. This can be described as the avoiding the wrong hell dilemma. If you happen to follow the right religion, you may indeed go to heaven and avoid hell. However, if you choose the wrong religion, youll still go to hell.
The thing missed by so many who use this argument is that you cannot bet on the general concept of theism. You have to pick specific doctrines. Theism is just a broad construct which includes all possible god-beliefs and, as such, does not exist absent specific theologies. If you are going to really believe in a god, you have to believe in something which means picking something. If you pick nothing, then your belief is literally empty and you remain an atheist. So, a person who picks risks picking the wrong god and avoiding the wrong hell.
A second problem is that it isnt actually true that the person who bets loses nothing. If a person bets on the wrong god, then the True God (tm) just might punish them for their foolish behavior. Whats more, the True God (tm) might not mind that people dont bother believing in it when they use rational reasons thus, not picking at all might be the safest bet. You just cannot know.
Also, some choices do indeed come with large risks. Many have died because they trusted in prayer rather than medicine. Others have perished due to the handling of poisonous snakes and the drinking of lethal liquids because Jesus said they would be able to do so without harm. Thus, the choice of pseudoscientific and mystical beliefs can carry very negative consequences.
A third problem is the unstated premise that the two choices presented are equally likely. It is only when two choices are equal in probability that it makes sense to go with the allegedly safe bet. However, if the choice of a god is revealed to be a great deal less likely than the choice of no god, then god ceases to be the safe bet. Or, if both are equally likely, then neither is actually a safe bet.
One final problem is the conclusion of the argument, where a person decides to believe in a god because it is the choice that offers the most benefits and least dangers. However, this requires that the god in question not mind that you believe in it merely in order to gain entrance to heaven and/or to avoid punishment in hell.
But this means that this god isnt actually a just or fair god, since a persons eternal fate is not being decided upon based on their actions, but merely on their decision to make a pragmatic and selfish choice. I dont know about you, but that certainly isnt the sort of god I would ever consider worshipping.