The Principle of Contingent Causation: The Impossibility of Infinite Regress.

[Belacqua]

If a non-contingent creator "hasn't come to be, by definition", then you are actually arguing that Thomas was wrong ... just saying. This is how incoherent you're being.

No, it's a key part of Thomas's argument that anything which is non-contingent has not come to be. And this includes, of course, a First Cause. He thinks that the First Cause is the only non-contingent thing.

Do you think that Thomas argues that God came to be? The whole point is that he never came to be, he is the only non-contingent, uncreated thing.

You've just argued above that non-contingent beings haven't come to be and now 50 words later you're arguing again they they are required. Might you make up your mind?

"Contingent' means that they depend for their existence on something else. "Non-contingent" means that they don't. The non-contingent First Cause (which Thomas says is the only non-contingent thing) hasn't come to be. 

You should check out some quantum mechanics. You might learn a little.

Currently it appears that some quantum events occur without an efficient cause. As I explained earlier in the thread, the causes that Aristotle and Thomas talk about are not only efficient causes. 

To repeat: for them, the causes of X are all the things that must be the case in order for X to be the case. All quantum events depend for their occurrence on other things being the case -- for example, the existence of space/time. Therefore, in the sense of αἰτία, which is what Thomas is talking about, all quantum events are caused.

Now explain to me again how a non-contingent being doesn't exist at all, but is required for contingent beings.

Thomas argues that one non-contingent thing does exist, but hasn't come to be. It couldn't come to be, because there is nothing essentially prior which could cause it to come to be. 

"Come to be" is not the same as "exists." It means "begins to exist." Thomas thinks that God exists, but never came to be.

You added the idea that it has to last forever, which is of course a Christian belief, but isn't covered in the Argument from Contingency. 

As to why the non-contingent thing is necessary for contingent things to exist, that's what the thread is about, and it's been covered several times already.