The true foundation
A good foundation is essential to any building. A house built on a solid foundation can withstand the attacks that are sure to come, but a poor foundation causes the entire structure to weaken, suffer damage, and even collapse. The only sure foundation for our spiritual lives is the word of God (Mt 7:24-27). Jesus Christ is the Word made flesh (Jn 1:14, Rev 19:13). As such, He is the foundation the church is built on (Is 28:16, Mt 21:42-44 with Ps 118:22 and 1Pet 2:4-8, 1Cor 3:9-17, Eph 2:20). Any spiritual endeavor built on another foundation is false.
When Jesus was in Caesarea, He asked the apostles who men were saying He is (Mt 16:13-18). Some said He was John the baptizer, some Elijah, some Jeremiah, and some one of the prophets. These were not trite things. John had been beheaded and buried without his head (Mt 14:6-12). Elijah had ascended to heaven over 800 years earlier (2Ki 2). Jeremiah had been dead about 600 years. The last book of the Bible had been written over 400 years before. Any one of these possibilities would have been miraculous, but the truth was even greater.
Jesus then asked His disciples who they said He is. Peter answered and said, " Thou art the Christ, the Son of the living God." Jesus replied, "Blessed art thou, Simon Barjona: for flesh and blood hath not revealed it unto thee, but my Father which is in heaven. And I say also unto thee, That thou art Peter, and upon this rock I will build my church; and the gates of hell shall not prevail against it." Peter is not the foundation Jesus was talking about. The word translated Peter is petros which means a piece of rock. Jesus used the word petra to describe the foundation of the church. Petra mean mass of rock, a large stone. Peter is not the foundation of the church. Jesus is, and in particular the Lord was referring to the revelation Peter had about the identity of Jesus Christ.
Notice Peter did not say, " Thou art God the Son." That phrase is nowhere in the Bible. The Son is not a divine person in the Godhead. It is the humanity God the Father manifested Himself in (Jn 1:1-3 & 14, Jn 14:7-11, 2Cor 5:19, Col 2:9, 1Tim 3:16). God is a Spirit (Jn 4:24), and a spirit does not have flesh and bones (Lk 24:39). The Father is the Spirit, the divine, the Godhead. The Son is the flesh, the man. God is not only not 3 persons, He is not even one person. The definition of a person is "the body of a human being." The only person was the man Christ Jesus (1Tim 2:5). The Son was begotten. Begotten and eternal are opposite. God did not have a body before Jesus was conceived in time in the womb of Mary (Ps 110:1, Gal 4:4, Heb 10:5).
The singular word person appears 3 times in the KJV in relation to God (Job 13:8, 2Cor 2:10, and Heb 1:3). In Job it is the Hebrew word paniym which literally means face. This is the same word God spoke to Moses in Ex 33:20-23. First, this is not a literal face. God did not have a body before Jesus was conceived (Ps 110:1, Heb 10:5). God is everywhere (Ps 139:7-10). God was telling Moses he could not handle the fullness of God’s direct presence. Second, it’s still only one, not three. In 2Cor 2:10 it is the Greek word prosopon which also means face. We can see the light of the knowledge of the glory of God in the face of Jesus Christ (2Cor 4:6). It could also be interpreted as countenance or presence. Again, it is one person and not three. In Heb 1:3 it is the Greek word hupostasis, which means essence or substance. Once again, there is nothing of co-equal persons. Rather, these verses show God revealed in one person - the Lord Jesus Christ.
Thus, what Jesus was telling Peter was that the revelation that Jesus Christ is the only true God manifested in the flesh is the foundation He was going to build His church on. Any religious group built on any other foundation is not the church Jesus Christ is building. He said, "I said therefore unto you, that ye shall die in your sins: for if ye believe not that I am he, ye shall die in your sins (Jn 8:24)." The word he in the phrase "I am he" is italicized, which means it was not in the original Greek. Jesus was saying that unless we believe that He is the I AM that met Moses in the burning bush (Ex 3:14), Jehovah God the Father, Creator, and Lord of all, there is no other way to be saved. Having the revelation of the true identity of Jesus Christ is essential to our relationship with Him (1Jn 2:18-25, 2Jn 7-11). How can we have a proper relationship with Him when we do not even understand who He really is? This is a revelation just like it was for Peter. We cannot figure it out with our intellect. It is the foundation of everything else the church is built on.