Explain how something that never began to exist can happen to exist. All of the things I can think of that never began to exist stay non-existent.
Clearly something must be eternally existent, the so-called "Ground of Being," or else you have the problem of an infinite regression of causes.
If your alternative to this "problem" is impossible, then you haven't solved the problem (to the extent that it's a problem at all - why do you think it is?). Is it possible for a being that didn't begin to exist to exist? You've given no reason to conclude so. Until you've eatablished that the conclusion you want to jump to is even possible, you can't be justified in jumping to it.
Another way of looking at it:
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you can't see how things could exist without tracing everything back to something that didn't begin to exist.
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I can't see how something that didn't begin to exist can exist.
If we're using personal incredulity as a substitute for rational arguments, then your explanation fails, too. However, if we set our incredulity aside and try to base our positions on logical deductions from facts, then you have more work to do: your position can't be taken as some sort of default. It has to be supported on its own merits.
More than one is possible, if they are also all one. Hence the doctrine of the Trinity.
Why did you add "if they are also all one?" Where is this coming from?
To elaborate a little, the Christian position is that God is three in person but one in being. Because each person of the Trinity is infinite, and because all true infinities are infinitely overlapping, the being-ness of the Father is not separable from the Son, which is not separable from the Holy Spirit. It's sort of like a three-way Venn diagram where each of the circles stretches out forever.
I think you missed my point.
We start with the question "is it possible for a thing to exist if it never started to exist?" Your answer, apparently, is yes. The implication here is that some number of these things exist.
The next question for the theist is "
do such things exist?" - to establish the existence of your god, you need to elevate your god from merely possible to actual.
If we're going for monotheism, the next question is "is there any sort of limit on the number of such things?" You've asserted "yes" and that this limit is one (or one trinity), but haven't given any explanation as to why.