So, is that you back again, Xavier?  Using the name "DeusVult" to suggest that you're here on some kind of crusade?  

So, I'm not sure I follow the argument, as there's almost a phenomenological bent to it where you're claiming that something cannot exist if it is not known, i.e. where the knowledge / perception of reality effectively determines reality ... except obviously you're referring to God rather than individuals.

I think that the same individuals who believe that nothing can become something or that an invisible something can have an eternal existence, are also not going to buy this line of argument.  In fact, they're not going to buy any argument ... and so I consider these types of proofs for the exitence of God to be a waste of time.  Now, the reason they don't accept the existence of God is simply due to bad will.  They don't want there to be a God, so no argument you can make is going to change their minds, short of perhaps God appearing directly to them.

Consequently, I always take the Pascal's Wager approach, along the following lines:

"You say that I can't prove the existence of God, so there is no God.  But that's a logical fallacy.  Even if it's true that I can't prove it, that does not disprove it.  By your own criteria you can no more DISprove the existence of God than I can prove His existence.  You should at least be honest with yourself and admit that you simply don't know, that you don't know there is God or that there isn't God, and that you can't prove either one.  Now, with this in mind, if you're right and there is no God, when I die I won't even know ... or at least won't care ... that I was wrong.  If I'm right, and you spent your entire life denying Him, it might not end up so good for you, especially if this God will also punish the wicked.  So the downside to your being wrong is infinitely greater than the downside to my being wrong.  If you can be honest enough to admit that you don't know, and if you sincerely desire to know the truth, one way or the other, just call out to Him in all sincerity to help you to know that He exists."

If someone can at least get to that point of sincerity of admitting they don't know (rather than denying Him simply because they don't WANT there to be a God, usually so they can continue living in sin with impunity), and asking God to help them know that He exists and to believe in Him ... God will grant that prayer.

OP says
If there are things that exist, God must necessarily be

Instead of our own conjectures, it's always best to rely on the Church.
Read St. Thomas Acquinas' Five Proofs for details on this.

St. Mary's Press
The Summa Theologica is a famous work written by Saint Thomas Aquinas between AD 1265 and 1274. It is divided into three main parts and covers all of the core theological teachings of Aquinas’s time. One of the questions the Summa Theologica is well known for addressing is the question of the existence of God. Aquinas responds to this question by offering the following five proofs:
1. The Argument from Motion: Our senses can perceive motion by seeing that things act on one another. Whatever moves is moved by something else. Consequently, there must be a First Mover that creates this chain reaction of motions. This is God. God sets all things in motion and gives them their potential.
2. The Argument from Efficient Cause: Because nothing can cause itself, everything must have a cause or something that creates an effect on another thing. Without a first cause, there would be no others. Therefore, the First Cause is God.
3. The Argument from Necessary Being: Because objects in the world come into existence and pass out of it, it is possible for those objects to exist or not exist at any particular time. However, nothing can come from nothing. This means something must exist at all times. This is God.
4. The Argument from Gradation: There are different degrees of goodness in different things. Following the “Great Chain of Being,” which states there is a gradual increase in complexity, created objects move from unformed inorganic matter to biologically complex organisms. Therefore, there must be a being of the highest form of good. This perfect being is God.
5. The Argument from Design: All things have an order or arrangement that leads them to a particular goal. Because the order of the universe cannot be the result of chance, design and purpose must be at work. This implies divine intelligence on the part of the designer. This is God.

OP says
If there are things that exist, God must necessarily be

Well, that's precisely what St. Thomas says also, in the very First Cause argument you cited.

But OP's angle is not from causality, but from knowability, where he states that something that exists must be knowable, and therefore must be known (a short paraphrase that probably gets his reasoning wrong).

Unless I misunderstand it (perhaps OP an help clarify), it almost sounds like a premise is that things can only exist insofar as they were known, i.e. where knowability is what creates existence, with the corollarly almost being if something isn't known, it doesn't exist, which is the phenomenological position that classic Aristotelianism / Thomist would reject.

Well, that's precisely what St. Thomas says also, in the very First Cause argument you cited.

But OP's angle is not from causality, but from knowability, where he states that something that exists must be knowable, and therefore must be known (a short paraphrase that probably gets his reasoning wrong).

Unless I misunderstand it (perhaps OP an help clarify), it almost sounds like a premise is that things can only exist insofar as they were known, i.e. where knowability is what creates existence, with the corollarly almost being if something isn't known, it doesn't exist, which is the phenomenological position that classic Aristotelianism / Thomist would reject.

I don’t know how to clarify more. In fact, nothing ever existed without being known by God, so « in posteriori », it’s true. But I recognize that my argument is not based on causality, that’s why I said maybe it’s invalid.

Certainly, WE know that nothing ever existed withotu being known by God, but that somewhat presumes the existence of God to begin with.  So, for someone who doesn't believe that there is a God, and that's their starting point, this wouldn't be particularly persuasive.  As I said, though, those who don't believe in God (or say they don't and try to convince Himself that He doesn't exist), they behave this way because they do not WANT to believe in Him.  So the issue isn't typically to convince them intellectually (since they won't be convince that way), but for the grace of God to change their hardened bad wills.

For sure most don’t want to to believe in God, but some only don’t have sufficient arguments to be sure. That’s for there people that we give arguments.