John 1:34

κἀγὼ ἑώρακα … ὁ υἱὸς τοῦ θεοῦ. “And I have seen and have testified that this is the Son of God.” The Synoptists tell us that a voice was heard at the baptism declaring “this is my beloved Son”; and in the Temptation Satan uses the title. Nathanael at the very beginning of the ministry, and the demoniacs very little later, use the same designation. This was in a rigidly monotheistic community and in a community in which the same title had been applied to the king, to designate a certain alliance and close relation between the human representative and the Divine Sovereign. Whether the Baptist in his peculiar circumstances had begun to suspect that a fuller meaning attached to the title, we do not know. Unquestionably the Baptist must have found his ideas of the Messianic office expanding under the influence of intercourse with Jesus, and must more than ever have seen that this was a unique title setting Jesus apart from all other men. The basis of the application of the title to the Messiah is to be found in 2 Samuel 7:14, “I will be to him a Father and he will be to me a Son”. In the second and eighty-ninth Psalms the term is seen passing into a Messianic sense, and that it should appear in the N.T. as a title of the Messiah is inevitable.