Why was it necessary for God the Son to come incarnate as opposed to God the Father or God the Holy Spirit?
They did come… together they are one God. To see Jesus is to see the Father.
Yes, but only the second person of the Trinity has a human body now for eternity. What is it about the Son that made him the suitable person to receive this body by becoming human?
I believe it was because so we could become adopted as sons of God in Christ…if the Father had become incarnate would - when we believed & were baptised into Him - become the Father of God the Son because we were ‘in the Father’? Of course that sounds heretical & just wrong, and so I suppose this is a stupid question but does anybody else like to consider the ins & outs of the Trinity & God’s plan of salvation even though it is too much for us to grasp?!
Or maybe it was because God, knowing human weakness, wanted us to identify WITH God the Son who receives the overflowing love of the Father, which elevates mankind to the life of the Trinity
You are right: the mystery of the Trinity is too much for us to grasp.
It might help to consider that sons always come from fathers, not the other way around. It would probably appear very odd to the Jewish people for God the Father in human form to call up to his Son in submission.
Father, you sent your Word to bring us truth and your Spirit to make us holy. Through them we come to know the mystery of your life.
John 4 21-26
Jesus saith to her: Woman, believe me, that the hour cometh, when you shall neither on this mountain, not in Jerusalem, adore the Father. You adore that which you know not: we adore that which we know; for salvation is of the Jews. But the hour cometh, and now is, when the true adorers shall adore the Father in spirit and in truth. For the Father also seeketh such to adore him. God is a spirit; and they that adore him, must adore him in spirit and in truth. The woman saith to him: I know that the Messias cometh (who is called Christ); therefore, when he is come, he will tell us all things. Jesus saith to her: I am he, who am speaking with thee.
St. Anselm dealt with this.
Anselm. If one of the other persons be incarnated, there will be two sons in the Trinity, viz., the Son of God, who is the Son before the incarnation, and he also who, by the incarnation, will be the son of the virgin; and among the persons which ought always to be equal there will be an inequality as respects the dignity of birth. For the one born of God will have a nobler birth than he who is born of the virgin. Likewise, if the Father become incarnate, there will be two grandsons in the Trinity; for the Father, by assuming humanity, will be the grandson of the parents of the virgin, and the Word, though having nothing to do with man, will yet be the grandson of the virgin, since he will be the son of her son. But all these things are incongruous and do not pertain to the incarnation of the Word. And there is yet another reason which renders it more fitting for the Son to become incarnate than the other persons. It is, that for the Son to pray to the Father is more proper than for any other person of the Trinity to supplicate his fellow. Moreover, man, for whom he was to pray, and the devil, whom he was to vanquish, have both put on a false likeness to God by their own will. Wherefore they have sinned, as it were, especially against the person of the Son, who is believed to be the very image of God. Wherefore the punishment or pardon of guilt is with peculiar propriety ascribed to him upon whom chiefly the injury was inflicted. Since, therefore, infallible reason has brought us to this necessary conclusion, that the Divine and human natures must unite in one person, and that this is evidently more fitting in respect to the person of the Word than the other persons, we determine that God the Word must unite with man in one person.
Our Lord Jesus was already the Son, and so to come as the Son of God and to direct us to the Father would only make sense. In addition, one of the most loving actions was to sacrifice your one and only son (as with Abraham and Isaac). Therefore, for God the Father to sacrifice His Son, now with full human nature along with His divine nature, would be the love and the perfect Sacrifice needed to atone for all the sins of the world.
May God bless you all!
St. John connects the Word becoming man to Genesis. He wrote:
 In the beginning was the Word, and the Word was with God, and the Word was God.
 He was in the beginning with God;
 all things were made through him, and without him was not anything made that was made.
 In him was life, and the life was the light of men.
Genesis 1 says several times: “And God said…” and with each Word, a new thing was created. That Word is the Son of God.
St. Paul echoes this:
Col. 1 He is the image of the invisible God, the first-born of all creation;
 for in him all things were created, in heaven and on earth, visible and invisible, whether thrones or dominions or principalities or authorities – all things were created through him and for him.
 He is before all things, and in him all things hold together.
The Father spoke (the Word) and all things came to be. So, Jesus is God’s Word–the one who directly interacts with all God’s creatures, sent by the Father to be the “image of God” with whom we can identify.
Theologians tell us that the Presence in the burning bush was the Word, who spoke to Moses God’s will for him. Some believe that one of the three visitors Abram entertained was the Word appearing in human form (perhaps pre-existing as his Incarnated self living in eternity).
Aquinas deals with this topic in the Tertia Pars of the Summa Theologica.
It is most directly dealt with in Question 3
The best explanation of the Trinity I have yet found is here:
It’s not too long and easy to understand.
This is the perfect explanation. Saint Thomas Aquinas is such a great resource for the hard questions.
Roles of Persons of the Trinity…?
Again, however, the Trinity is hard to grasp. Some Binitarians (members of “apostates” from the World Church of God, in this case) believe that Jesus is the Holy Spirit incarnate – but, that is easily refuted with the question of why they called Him the Son!
It states that “In Catholic theology, the Holy Spirit is said to proceed from the will of both the Father and the Son”, however the Father, Son, and Holy Spirit, have only one will and one mind.
The Incarnate God is highly systematic. His holiness improves by the level of intelligence of the ground.