For unto us a child is born, unto us a son is given: and the government shall be upon his shoulder: and his name shall be called Wonderful, Counsellor, The mighty God, The everlasting Father, The Prince of Peace.” Isaiah 9:6.

This passage proves the trinity because Christ is called the "mighty God" and the "everlasting Father"!

Answer:

Much stress is laid on Isaiah 9:6, as proving a trinity because Christ is called the everlasting Father. But it can have no reference to a trinity. Is Christ the Father in the trinity? If so, how is he the Son? Or if he is both Father and Son, how can there be a trinity? For a trinity is three persons. To recognize a trinity, the distinction between the Father and Son must be preserved. Christ is called “the second person in the trinity” but if this text proves a trinity, or refers to it at all, it proves that he is not the second, but the first. And if he is the first, who is the second? It is very plain that this text has no reference to such a doctrine. And, as we compare Scripture with Scripture, we can be sure that it does not support the “Jesus only” doctrine.

The Son is referred to as the everlasting Father, not of Himself, nor of His Father, but of the children which his Father has given him. His language is “I and the children which God hath given me.” Hebrews 2:13. Paul is quoting Isaiah 8:18 and applies it to Christ.

Furthermore, Jerusalem is “the mother of us all” Galatians 4:26, and she is the bride of her Husband Jesus Christ (Revelation 21:2). Thus if Jesus is the husband of our mother, this makes Him our everlasting Father.

Notice also that Isaiah 9:6 says that “his name shall be called… The mighty God.” This term emphasizes the divine nature of Christ. He is indeed mighty, for all power is given unto Him (Matthew 28:18) and He is divine by virtue of His divine birth (Hebrews 1:4, 8). It is therefore appropriate to refer to the Son as mighty, for he is powerful. It is also appropriate to refer to him as God, for the Most High God himself refers to his Son as God in Hebrews 1:8. Therefore the terms “everlasting Father” and “The mighty God” can rightly apply to the Son.

Spirit of Prophecy:
“However much a shepherd may love his sheep, he loves his sons and daughters more. Jesus is not only our shepherd; He is our "everlasting Father." And He says, "I know Mine own, and Mine own know Me, even as the Father knoweth Me, and I know the Father." John 10:14, 15, R. V. What a statement is this!--the only-begotten Son, He who is in the bosom of the Father, He whom God has declared to be "the Man that is My fellow" (Zech. 13:7),--the communion between Him and the eternal God is taken to represent the communion between Christ and His children on the earth!” {EGW, DA 483}