Q. How do you prove it?
A. John 14:9,10,11, Jesus saith unto him, Have I been so long time with you, and yet have you not known me, Philip? He that has seen me has seen the Father; and how do you say then, Show us the Father? Do you not believe, that I am in the Father, and the Father in me? The words that I speak unto you I speak not of myself: but the Father that dwells in me he does the works. Believe me that I am in the Father and the Father in me. Secondly, and being a personage of tabernacle, was made or fashioned like unto man, or being in the form and likeness of man. [§5. ¶2.] Philip. 2. Let this mind be in you, which was also in Christ Jesus; who, being in the form of God, thought it not robbery to be equal with God; but made himself of no reputation, and took upon him the form of a servant, and was made in the likeness of man, and, being found in fashion as a man, he humbled himself, and became obedient unto death, even the death of the cross. Heb. 2. 14, 16. Forasmuch then as the children are partakers of flesh and blood, he also himself likewise took part of the same. For verily he took not on him the nature of angels: but he took on him the seed of Abraham. Thirdly, he is also in the likeness of the personage of the Father. [§5. ¶2.] Heb 1. 1,2,3. God, who at sundry times, and in divers manners, spake in times past to the fathers, by the prophets, hath in these last days spoken unto us by his Son, whom he hath appointed heir of all things, by whom also he made the worlds; who being the brightness of his glory, and the express image of his person. Again, Philip. 2:5,6 Let this mind be in you, which was also in Christ Jesus; who, being in the form of God, thought it not robbery to be equal with God.
Q. Was it by the Father and the Son that all things were created and made that were created and made?
A. It was. Col 1:15,16,17 Who is the image of the invisible God, the first born of every creature; for by him were all things created that are in heaven and that are in earth, visible and invisible, whether they be thrones or dominions, principalities or powers; all things were created by him and for him; and he is before all things, and by him all things consist. Gen. 1:1 In the beginning God created the heavens and the earth. Heb. 1:2 [God] Has in these last days spoken unto us by his Son, whom he has appointed heir of all things, by whom also he made the worlds.
Q. Does he possess the fullness of the Father?
A. He does. Col. 1:19. 2:9. For it pleased the Father that in him should all fullness dwell. For in him dwelleth all the fullness of the Godhead bodily. Eph 1:23. Which is his [Christ's] body, the fullness of him that fills all in all.
Q. Why was he called the Son?
A. Because of the flesh. Luke 1:33 That holy thing which shall be born of thee, shall be called the Son of God.—Math. 3:16,17 And Jesus, when he was baptized, went up straightway out of the water: and lo, the heavens were opened unto him, and he [John] saw the Spirit of God descending like a dove and lighting upon him: and lo, a voice from heaven, saying, This is my beloved Son, in whom I am well pleased.
Q. Was he ordained of the Father, from before the foundation of the world, to be a propitiation for the sins of all those who should believe on his name?
A. He was. 1 Peter 1:18,19,20: Forasmuch as ye know that ye were not redeemed with corruptible things, as silver and gold, from your vain conversation, received by tradition from your fathers; but with the precious blood of Christ, as of a lamb without blemish and without spot: who verily was foreordained before the foundation of the world, but was manifested in these last times for you. Rev. 13:8 And all that dwell upon the earth shall worship him [the beast] whose names are not written in the book of life of the Lamb slain from the foundation of the world. 1 Corin. 2:7 But we speak the wisdom of God in a mystery, even the hidden mystery, which God ordained before the world unto our glory.