This window shows John the Baptist, who hoists above his head a scroll upon which is written in Latin: Ecce Agnus Dei, "Behold the Lamb of God." This is the prophetic description of Jesus Christ, the one who is "The Lamb of God who takes away the sin of the world" (John 1:29). John carries a staff in the shape of a cross.
John was a dramatic preacher who predicted that the Kingdom of God was coming soon. He called people to repent and be baptized. When he baptized Jesus, he saw the Holy Spirit descend upon Jesus in the form of a dove, and heard God say, "This is my beloved Son, with whom I am well pleased."
"The phrase Lamb of God comes from the Old Testament sacrifices. Sin puts every individual under sentence of death, which is literally separation from God. But in Old Testament times, God accepted the death of an animal as substitute for the death of a person. Further sin meant repeated sacrifices. Jesus died to give his life once and for all, sacrificed like a lamb for human sin throughout the ages."
-p. 534, Eerdman's Handbook to the Bible, 1973
The Purves Window was presented in memory of George Tybout Purves (1852-1901), the sixth minister of First Presbyterian Church of Pittsburgh who served in 1886.