Kerygma & Trinity
Kerygma (Greek: κήρυγμα, kérugma) is the Greek word used in the New Testament for preaching (see Luke 4:18-19, Romans 10:14, Matthew 3:1). It is related to the Greek verb κηρύσσω (kērússō), to cry or proclaim as a herald, and means proclamation, announcement, or preaching. Preaching or proclaiming, is distinct from teaching or instruction (didache) in the Gospel of Christ. Before the Gospel was written, it was first preached (Romans 16:25), but beyond preaching it was also to be taught (Matthew 28:19) in order that, as far as possible, it might be understood (Matthew 13:19).
The New Testament teaches that as Jesus launched his public ministry he entered the synagogue and read from the scroll of Isaiah the prophet. He identified himself as the one Isaiah predicted in Isaiah 61 (Luke 4:17-21). The text is a programmatic statement of Jesus' ministry to preach or proclaim (Kerygma), good news to the poor and the blind and the captive.
If one carefully considers the early kerygma, a Trinitarian theology is implicit therein. As early as twenty years after Jesus' death, the great Christ-hymn of the Letter to the Phillipians (cf. Phil 2:6-11) offers us a fully developed Christology stating that Jesus is equal to God, but emptied Himself, became man, and humbled himself to die on the Cross, and that to Him now belongs the worship of all creation, the adoration that God, through the Prophet Isaiah, said was due to Him alone (cf. Is 45:23).
The earliest professions of faith in the Apostolic Church are Christological and expressed in concise formulas: 'Jesus is the Christ' (cf Acts 2:36; 10:36; Col 2:6); 'Jesus is the Lord' (1 Cor 12:3; Rom 10:9; cf Acts 2:36; Phil 2:11); 'Jesus is the Son of God' (cf Acts 9:20; 13:33; Rom 1:4; Heb 4:14). Soon it received a more ample development in which the Christ-event, the central event of salvation history, is progressively elaborated upon (1 Cor 15:3-4; Phil 2:6-11; 1 Tom 3:16). A further development in the life of the Apostolic Church is the introduction of a Trinitarian profession of faith. This is a natural evolution, not some orchestrated decision passed at council by means of assassination or coercion of any sort (if one studies the process and development it takes place universally), for the Trinitarian confession was latent in the Christological (cf. Acts 2:33) and clearly implied in the early kerygma (cf. Acts 2:14-39; 3:12-26; 4:8-12; 5:29-32; 10:34-43; 13:16-41). The Trinitarian profession of faith in the New Testament is best witnessed to by Matthew 28:19-20 and 2 Cor 13:13; it corresponds to the Trinitarian teaching of the Apostles (cf. Ephesians 1:3-14).
So, to sum up, the Trinitarian expression of faith was implied in the first preaching and is evident throughout the Old Testament.