I find it interesting how the first year philosophy student asking such a non-question automatically correlates the philosophy of religion with Christianity. As if.

(an omnipotent god)

This is not like asking god, or a human, to make a "square circle".

Can god make a snowball he can't lift? Humans can do something god can't.

Can god make a more powerful god? Suicide? Evolve? Grow?

A better theist answer might be that god could make a snowball and then not be able to lift it, until he wants to. Since god's goal is failure, he is still omnipotent. Or why not solve this with a creative use of time as another demension - god exists without time (how else could be made in the first place-he was never made at all!) . So god creates a rock he cannot lift and lifts, but not at the same time - because there is no time!

It's self-contradictory as long as god is defined to be forever omnipotent (ie infinite power). A rock that god cannot lift cannot exist (illogical) because god can always lift any rock he makes. If god could make this rock, it would prove he was not omnipotent therefore, since god is defined with infinite power, the rock cannot exist.  In this way it's like the square-circle. Though, the question is less obvious that it's illogical than a square-circle. Of course if god is illogical and omnipotent - he really can do whatever he wants.

If god's power is finite, then can he make a rock he cannot lift? Would you define this rock as a square-circle? If god's power is finite, the rock can exist - even if god cannot make it.

Though, somewhere I read that god never changes, does this mean his power is finite? Would an increase in power cause a decision to be changed, his character to be changed?

Good discussion, guys.  Keep it up - the church needs to continue to engage such interesting dialogue!

 I think Garry understands the situation here - that's it's like a square-circle just in case we understand omnipotence as "infinite power" (though I would have issues with this particular definition, it's sufficient for our purposes here).  What needs to be corrected is what it means to be self-contradictory.  Now, keep in mind it isn't objects that are self-contradictory but propositions.  That the atheistic objection boils down to an omnipotent God creating something that limits that power shows, in a more obvious way, the inconsistent nature of the "thing" to be created.  If this does not strike one as "obvious" like a square-circle, this might be because notions of "squares" and "circles" are descriptions more prevalent in our day-to-day understandings.  Therefore it is correct to say that if God were not omnipotent, then such a thing could conceivably exist.  But that also means that neither proposition contradicts the other ("God is finite" does not contradict "something God cannot do").

One final remark - Malachi 3:6a.  The passage here is about God's fidelity and character, not about his omnipotence (cf. Hebrews 13:8).  But to attach a better understanding of omnipotence, it would be better defined as, in the words of Thomas Flint and Alfred Freddoso, "maximal power" (see their important seminal essay at http://www.nd.edu/~afreddos/papers/mp.htm).