What follows is the argument I personally find the most convincing to support the existence of a deity; but I mean it is the best argument - I don't mean that an argument is the best reason for believing in god. (It isn't.)
And it is not an argument for specifically the Christian God, nor for just one god - but it is a particular argument for a particular kind of deity.
If it was not for god, then we could not know anything about anything.
'Revelation' - direct communication from god - is the basis of all possible true knowledge.
The reason, is that real knowledge is a two-sided thing - there is a communication and something which is capable of receiving the communication - there is revelation and there is reception of that revelation.
This means that true knowledge requires a true message - which requires god, because only a deity could know the truth and communicate it; and also that Men have the capacity to receive truth; which requires that this capacity was 'implanted' in Men by god.
If god did not have a hand in 'implanting' the capacity to receive truth into Man, then there would be no reason to believe that Man could receive truth. There is no reason at all to assume that a being that had arisen by accident, or by evolution/ natural selection, would be able to know truth.
Natural selection merely explains how an organism may process information to enhance its reproductive success - this has nothing to do with truth. The 'knowledge' arising from natural selection is purely expedient, contingent, and retrospective - not 'true'.
And if natural selection did, by some chance, lead to an ability to perceive truth - then this could never be known; because truth would be evaluated by perceptual and cognitive systems which had evolved due to their reproduction-enhancing qualities - not due to their truth-recognizing qualities.
(If you cannot grasp this logical point about natural selection having nothing necessarily to do with truth, and wholly naturally selected entities being in principle unable to recognise truth, then it is unlikely you will understand this argument. So this may be a point to pause for thought.)
So, for us to know anything about anything - it is necessary that a deity communicated knowledge to us, and that the deity had a hand in making us capable of understanding knowledge in a full and relevant sense of understanding.
So, there must be revelation, and revelation must be two-sided.
So a deity must exist, and the deity must be the kind of 'god' who wants or needs to communicate with Men, and also had a hand in the creation of Men.
That narrows things down quite a bit!
The most obvious response to this argument is on the lines of pointing out that there are many, many different versions of what god has revealed to man, a huge amount of disagreement, much uncertainty (for instance there are many religions, and many who disbelieve in any deity), and even one man can change his mind in that topic throughout his life: what seemed like a revelation may become regarded as an error, and vice versa.
The idea is that if god was indeed communicating with us, and had also made us capable of understanding revelation 'how come' there is so much disagreement over what is being communicated?
All I would point out is that this is a second order matter of pragmatics, which is logically unconnected with the main argument.
It is perfectly possible (indeed I believe it is true!) that there really be a god who really reveals true knowledge to all Men who he has made such that they are capable of understanding correctly that revealed knowledge - all this could be true - and yet it could also be true that that true and revealed knowledge be rejected, resisted, misunderstood, reversed, or in some way rendered inoperative - for whatever reason; and there may be many such reasons.
In other words, the truth of revelation is a metaphysical necessity for us to make sense of the world. It is about the necessary structure of reality for anything to make sense at all.
And the truth of revelation is therefore unconnected with our empirical observation or experience.
This argument leads to a picture of deity, of god, which is a long way short of the Christian god.
But the argument does have value, because it establishes revelation as the basis of all possible knowledge.
And that, for the typically confused, self-refuting modern atheist skeptic, may be a vitally important first step.
Because the mainstream atheist skeptic needs to know his beliefs are strictly and logically nonsensical - non-sensical, does not make sense - that he has zero basis for any belief in anything at all except by accepting the necessity of the principle of revelation.
Anyway, this is the argument I personally find the most-convincing one for establishing the existence of god - however I acknowledge that it is a difficult argument to follow, and an easy one not to understand or to reject on false grounds.
The argument certainly requires at least a few minutes of hard and focused thinking - and that is something way beyond the capacity of most modern people.
This 'revelation-reception' argument leads to an understanding that there is such as thing as truth and it can in principle be known.
In contrast, the mainstream modern atheist skeptic has a paradoxical understanding that that he personally knows something like the following:
1. There is no such thing as 'the truth';
2. But if there was, there is no way that anybody could know that they knew it;
3. And even if there was truth and somebody knew they knew it - they could not communicate this to anybody else (they could not know whether the truth had actually been communicated, and that the other person was capable of understanding the truth, and that the truth had actually been understood).
The modern atheist skeptic is only sure of one thing - which is that he never can be sure of anything (except that he never can be sure).
Of course, thoughtful skeptics recognize this simple and obvious paradox - however they see no way out of it.
Well there is a way out! - which is the above 'two-sided' argument.