A.T. Robertson Commentary Matthew 3:16

"And Jesus when he was baptized, went up straightway from the water: and lo, the heavens were opened unto him, and he saw the Spirit of God descending as a dove, and coming upon him;" — Matthew 3:16 (ASV)

The Spirit of God descending as a dove (πνευμα θεου καταβαινον ωσε περιστεραν). It is not certain whether Matthew means that the Spirit of God took the form of a dove or came upon Jesus as a dove comes down. Either makes sense, but Luke (Luke 3:22) has it "in bodily form as a dove" and that is probably the idea here. The dove in Christian art has been considered the symbol of the Holy Spirit.