The "JW's" are Arian heretics. A simple refutation of Arianism used by St. Augustine, St. Athanasius etc that the heretics could never answer was that the Son of God told us to be baptized in the "Name of the Father and of the Son and of the Holy Ghost" (Mat 28:19). Now, since He says Name, He teaches that the Three are One. This is the Name of the Trinity, the one Name of the Triune God. Arianism is a detestable heresy, condemned by all of Sacred Scripture. Were it not for the Protestant revolution and the disastrous principle of private interpretation it introduced, Arianism would never have reappeared. St. Peter, St. Paul, St. John, St. Thomas etc all teach Jesus is God and Creator, (2 Pet 1:1; Acts 20:28) Phil 2:6; Rom 9:5; Col 1:16-17; Jn 1:1;3; Jn 20:28), it was not a mere man, but God who redeemed us in His own blood, as these verses teach, and whoever does not know and believe that, and moreover is not baptized properly is not by any means a Christian, is a heretic, is yet in his sins, and is on the way to hell.
JW's assert an absurdity when they say that Jesus didn't create, contrary to these passages, or when they say He was supposedly a creature but was still instrumental in the creation of others. That is impossible and senseless, because no creature can create anything, nor be an intermediary in the act of creation, which only the Creator can accomplish and requires infinite power, since God brings a being into existence from non-being without a material cause. And Genesis clearly shows us God alone creates all things by His Word and His Spirit, just as St. John says. God's Word is in Him, and is inseparable from Him, just as is His Spirit. When God decides to create, He creates through His own intellect and will alone, His Word and His Spirit. When God says "Let Us make man", there is the plurality of the Godhead revealed right there in Gen 1:26. God conceives the object He wishes to create, and merely wills it into existence, He speaks the Word and they are called into being, He breathes His Spirit and they have life, as Psa 33:6 says.
The Name YHWH, as other posters have mentioned, describes God's eternal and self-subsistent nature. Jesus uses the same Name to describe Himself (Jn 8:24;8:58) and also declares faith in His divinity to be absolutely necessary for salvation. We know this by divine revelation, and we are bound to believe it to be saved. But although it is above reason, since no one can comprehend the Godhead, it is not opposed to reason, the Church who is infallibly guided by the Holy Spirit, and those whom She has approved can help us gain some understanding, as Vatican I declares, "Now reason, does indeed when it seeks persistently, piously and soberly, achieve by God's gift some understanding, and that most profitable, of the mysteries, whether by analogy from what it knows naturally, or from the connection of these mysteries with one another and with the final end of humanity; but reason is never rendered capable of penetrating these mysteries in the way in which it penetrates those truths which form its proper object." We believe in divine revelation because we are taught by the authority of Christ and His only true Church, not on what we judge or prefer to be true.
God is a Trinity, because He is eternal and self-subsistent. In a Being who is self-subsistent, St. Thomas proves at length, there can be no potentiality, nothing that God can become that He is already not. But if His intellect were not His Essence, His Intellect would be related to Him as act to potency. Therefore, His Intellect or His Word is His own Essence. And likewise, His Will or His Spirit is also His own Essence. God, His Word and His Spirit are One Being. The Word begotten of the Father alone, the Spirit proceeding from the Father and Son. A distinction of Person within the unity of Essence. The Holy Trinity. An analogy used by some of the Fathers was the light and heat proceeding from the sun, but in a way one with and inseparable from it.