In order to properly address this question, it will be helpful to distinguish between proof and persuasion, or—alternatively—between certainty as such and the right to be certain. Certainty, as it is being used in the question above, seems to be referring to a psychological state; people feel certain about all sorts of things under all sorts of circumstances. The more relevant question is not whether one can be certain of God’s existence, but whether such certainty can be justified.
Similarly, whether or not one is persuaded that God exists by a certain piece of reasoning can be due to a number of widely varying and complex factors, only one of which may be the actual piece of reasoning itself. The more interesting question, therefore, is whether there is proof that God exists—irrespective of whether anyone is persuaded by it.
The fundamental intuition underlying many proposed “proofs” for God’s existence is the idea that something cannot come from nothing. If this is true, then there are two possibilities: (1) Each thing comes from something else, forming a chain that extends forever into the past; or (2) All things are found “in” a single being, that is, in its power to bring them about. Such a being, if it existed, would be responsible for all that exists, and would thus plausibly count as God.
Consider the first possibility: while this scenario would explain the existence of each individual thing, what it would fail to explain is the chain itself; granted, all of the things that presently exist must receive their existence from something (if something cannot come from nothing), but why must each thing receive existence from the particular thing that it does? Why does this particular chain exist, rather than a different one? The chain itself counts as something, and since something cannot from nothing, the chain must have received existence from something as well.
It seems, then, that (2) is the only legitimate option. Of course, it is always open to the atheist to assert that something can (and does) come from nothing. At that point, however, one may appeal to the dictum that extraordinary claims require extraordinary evidence. If the atheist is truly committed to this principle, then he or she will have to offer extraordinary evidence for the extraordinary claim that something can come from nothing. One might be tempted to say that if there is another thing that's obvious, it is that this cannot be done.