Blaise Pascal, mathematician, philosopher, theologian, was the author of the common argument for God, coined "Pascal's Wager." It goes as follows:

• Your life is an essence a bet. You bet whether or not there exists an afterlife and a God that will judge your life. "Betting" that there is a God and being right, results in eternal life. "Betting" that there is a God and being wrong, means there is no significant loss.
• "Betting" that there is not a God, and being right, means no significant gain. "Betting" that there is not a God and being wrong, results in eternal damnation -- significant loss.

In more layman terms or as the street preacher often poses the question:

• What if you were to die today and stood before God, what would you say to him to let you into heaven?

To start with, Pascal's Wager is an emotional appeal (specifically a threat of hell for disbelief) that is intellectually inert, vacuous of actual argumentation, and does not flatter the speaker or listener. Christopher Hitchens has an excellent spread on it here:

Hitchens says of Pascal's Wager that it is:

Religious hucksterism of the cheapest, most vulgar, nastiest kind that is possible to imagine.

He also likens it to a slipshod debacle with a used car salesman. Altogether, Pascal's Wager is underhanded and devoid of logical argumentation.

Let me clarify, in case my audience thinks I exaggerate. Pascal's Wager rests on two grandiose assumptions. One, it assumes the asking party's deity is the one to bet on. But why can one not pose the same wager and substitute another god in the mix, such as Zeus or Kali or Satan, even? If the argument becomes persuasive for the Christian God (as was propositioned by Blaise Pascal), then why not a myriad of others? Extending this logic, if one bets on the Christian God, one should bet on all gods, and attempt to believe in them all, as a safety net. But how can one "bet," if you will, on all possible gods, especially if those gods contradict? Given the scenario where there exists a god that says following him requires not following other gods, who do you pick? How do you know that your belief has covered every god? Could there be a religion in the past that followed the correct god and you are unaware of that god now? In other words, what is to say still that you have "bet" on the right one?

Second, belief in this god (the betting on a god situation) cannot just be mustered up--in other words--one cannot just decide this is what they believe wholeheartedly. Such an action is really a deception, tricking oneself and those around about his or her beliefs. It is living a lie. A god that actually cared about people believing in him (so important that their disbelief means eternal punishment), would want real, actual belief, not a pretend, spontaneous feigning of belief. Also, if this god has any form of omniscience, he will see right through any pretend belief and still punish the betting man. On top of this, this says nothing about worship, which definitely cannot be faked in the mind of a god.

The argument boils down to a mere "what if you're wrong?" and provides no meaningful reason for why one should believe in a deity. That is what makes it vacuous and underhanded.