From Wikipedia, source of all knowledge:
- Matthew the Evangelist, the author of the first gospel is symbolized by a human (not an angel as sometimes thought) - a figure of humanity and reason. Matthew's gospel starts with Jesus' genealogy from Abraham; it represents Jesus' Incarnation, and so Christ's human nature. Christians should use their reason for salvation.
- Mark the Evangelist, the author of the second gospel is symbolized by a lion - a figure of courage and monarchy. Mark has John the Baptist preaching "like a lion roaring" at the beginning of his Gospel. It also represents Jesus' Resurrection (because lions were believed to sleep with open eyes, a comparison with Christ in the tomb), and Christ as king. Christians should be courageous for salvation.
- Luke the Evangelist, the author of the third gospel (and presumably the Acts of the Apostles) is symbolized by an ox bull or calf - a figure of sacrifice, service and strength. Luke's gospel starts on the temple duties of Zacharias in the temple; it represents Jesus' sacrifice in the Passion, and Christ as priest (this also represents Mary's obedience). Christians should be prepared to sacrifice themselves for salvation.
- John the Evangelist, the author of the fourth gospel is symbolized by an eagle - a figure of the sky, and believed to be able to look straight into the sun. John starts with an eternal overview of Jesus the Logos and goes on to describe many things with a "higher" level of theology than the other three "terrestrial" Synoptic Gospels; it represents Jesus' Ascension, and Christ's divine nature. Christians should look directly on eternity without flinching for salvation.
Happy Easter, Everyone!