St. John the Forerunner depicted as an angel comes from the quotation from Isaiah, found at the beginning of the gospel of St. Mark 'Behold, I send my messenger (Greek angelos) before thy face, which shall prepare thy way before thee.' The type originated in Byzantium in the 13th century and was popular throughout Asia Minor. St. John points to the diskos (eucharistic footed dish) held in his left hand. In it is the naked figure of Christ symbolising the Lamb of God of the Eucharist. 

Where we see the scroll the inscription is: Pokaitesya priblizhibosya tsaratvie nebosnoe ('Repent ye: for the kingdom of heaven is at hand'), Matthew 3:2.