We read in Matt., xvi, 15, 16: “Simon Peter answered and said: Thou art Christ, the Son of the living God. And Jesus answering, said to him: Blessed art thou, Simon Bar-Jona: because flesh and blood hath not revealed it to thee, but my Father who is in heaven.” The parallel passages have: “Thou art the Christ” (Mark 8:29), “The Christ of God” (Luke ,ix,20). There can be no doubt that St. Matthew gives the original form of the expression, and that St. Mark and St. Luke in giving “the Christ” (the Messias), instead, used it in the sense in which they understood it when they wrote, viz. as equivalent to “the incarnate Son of God” (see Rose, VI). Sanday, writing of St. Peter’s confession, says: "the context clearly proves that Matthew had before him some further tradition, possibly that of the Logia, but in any case a tradition that has the look of being original " (Hastings, “Dict. of the Bible”). As Rose well points out, in the minds of the Evangelists Jesus Christ was the Messias because He was the Son of God, and not the Son of God because He was the Messias.
How do you understand the bold part? How can this be demonstrated from scripture and the fathers?