Most people today consider Jesus Christ to have been a great man, but will not believe He is the unique Son of God. Nevertheless, He is indeed God's only begotten Son.
Before His birth, the angel Gabriel predicted it. "That holy thing which shall be born of thee shall be called the Son of God" (Luke 1:35). John the Baptist identified Him: "And I saw, and bare record that this is the Son of God" (John 1:34). His disciples recognized Him. "Nathanael answered and saith unto him, Rabbi, thou art the Son of God" (John 1:49). "Simon Peter answered and said, Thou art the Christ, the Son of the living God" (Matthew 16:16). John said that the very purpose of his gospel was to demonstrate "that Jesus is the Christ, the Son of God" (John 20:31). After Christ's resurrection, even Thomas exclaimed, "My LORD and my God" (John 20:28).
The powers of darkness grudgingly acknowledged who He was: "And unclean spirits, when they saw him, fell down before him, and cried, saying, Thou art the Son of God" (Mark 3:11). Even the centurion who supervised His crucifixion had to confess, "Truly this was the Son of God" (Matthew 27:54). Most important of all was the testimony of God the Father from heaven to Christ's Sonship, both at His baptism (e.g., Mark 1:11) and on the Mount of Transfiguration, as Peter records in our text.
Therefore it is necessary for our salvation that we also believe this. "He that believeth on him is not condemned: but he that believeth not is condemned already, because he hath not believed in the name of the only begotten Son of God" (John 3:18). HMM