‘And at once in the synagogues he proclaimed Jesus, that he is the Son of God.'
Saul immediately went to the synagogues one by one and proclaimed Jesus as the Son of God. The plural for synagogues suggests that minimally this took a period of weeks. He began a carefully planned tour of the synagogues on the Sabbaths that followed. Outside Acts ‘Son of God' or its equivalent is a title he regularly uses of Jesus (1 Thessalonians 1:10; Galatians 1:16; Galatians 2:20; Galatians 4:4; Galatians 4:6; Romans 1:3-4; Romans 1:9; Romans 5:10; Romans 8:3; Romans 8:29; Rom 8:32; 1 Corinthians 1:9; 1 Corinthians 15:28; 2 Corinthians 1:19; Ephesians 4:13; etc.). Galatians 1:16 suggests that the fact was vividly brought home to him by his experience on the Damascus Road. It was closely related to the idea of the Messiah (Psalms 2:7; Psalms 89:26; Matthew 16:16; Matthew 26:63; Mark 14:61), and the glory revealed to him there might well have brought home to him that Jesus was more than only the Messiah, something of which the Apostles must have been already aware, even though they had not yet worked out the detail (compare also Stephen's vision). He was the glorious Messiah, connected so closely with God that it gave new meaning to the term ‘Son of God', as Stephen had previously recognised. He was putting into words what Stephen saw.
It is important to recognise that the historical Jesus is central to his proclamation (Acts 9:27; Acts 17:7; Acts 17:18; Acts 19:13; Acts 20:21; Acts 28:23; Acts 28:31). He does not just think in terms of some mystical figure. And while in Acts he does not elsewhere use the term Son of God (but see Acts 13:33), Saul does consistently argue for Jesus' Messiahship and constantly stresses that he is the only source of salvation (Acts 13:23; Acts 16:31; Acts 17:3; Acts 18:5; Acts 19:4; Acts 26:18).
We have to smile when we consider his first entry into the synagogue. Here was the High Priest's official representative, bearing the High Priest's authority, and as he walked in he would be led to the special seats at the front. All would know why he was there. And then during the course of the gathering he would be asked to speak by the ruler of the synagogue, possibly even to read the Scriptures. And then he looked around at the gathered and expectant people - and began to proclaim that Jesus is the Son of God.