The day after baptizing Christ in the Jordan, John saw Christ and told his followers; "Behold the Lamb of God, which taketh away the sin of the world" (John 1:29). The artist here adopted one of the standard methods of indicating the topic of conversation - John addresses a crowd and points towards a cloud-frill (denoting a different ontological level) which contains the subject of his discourse; a conventional image of the 'lamb of God' (for more on this kind of visual deixis, see chapter 5 of my PhD thesis, partiularly section 5d.)