Rebirth as described by all the NT writers is initiated by belief in the seed message of Jesus’ Gospel of the Kingdom, as demonstrated by him in the parable of the sower (Mt. 13:19; Lk. 8:11-12; cp. 1 Pet. 1:23-25).
Acts 8:5 precisely defines the content of the saving Gospel as “proclaiming the Messiah.” In Acts 8:12 this is further clarified as “preaching the Gospel of the Kingdom,” while verse 4 describes the same activity as “preaching the word as gospel.”