But, that we may not have the appearance of intentionally passing by his charges through inability to refute them, we have resolved to answer each one of them separately according to our ability, attending not to the connection and sequence of the nature of the things themselves, but to the arrangement of the subjects as they occur in this book. Let us therefore notice what he has to say by way of impugning the bodily appearance of the Holy Spirit to our Saviour in the form of a dove. And it is a Jew who addresses the following language to Him whom we acknowledge to be our Lord Jesus: “When you were bathing,” says the Jew, “beside John, you say that what had the appearance of a bird from the air alighted upon you.” And then this same Jew of his, continuing his interrogations, asks, “What credible witness beheld this appearance? Or who heard a voice from heaven declaring you to be the Son of God? What proof is there of it, save your own assertion, and the statement of another of those individuals who have been punished along with you?”
Source: Against Celsus (New Advent)