http://www.newadvent.org/fathers/04161.htm
Celsus is assuming here that Christ is a spiritual being therefore he is addressing directly him: what proof is there of the fact that you (=presumed spiritual Christ) were declared son of God from heaven save the witness of the only man found during that proclamation?
Hence Celsus has found only two witnesses of the baptism of Christ:
- The spiritual Christ
- The witness of the mortal John
Clearly Celsus is not satisfied by these two witnesses because they are reduced to only one: the witness only of the man found there, when the Christ descended on himself, i.e. John.
Hence in the gospel read by Celsus, John was the only man who saw the dove and heard the voice from heaven.