Traditionally, the four Gospel writers have been represented by the following symbols: St. Matthew, a divine man; St. Mark, a winged lion; St. Luke, a winged ox; and St. John, a rising eagle.
These symbols are taken first from the Prophet Ezekiel (1:1-21):
“In the 30th year, on the fifth day of the fourth month, while I was among the exiles by the river Chebar, the heavens opened, and I saw divine visions. ... As I looked, a storm wind came from the North, a huge cloud with flashing fire, from the midst of which something gleamed like electrum. Within it were figures resembling four living creatures that looked like this: their form was human, but each had four faces and four wings, and their legs went straight down; the soles of their feet were round. They sparkled with a gleam like burnished bronze. Their faces were like this: each of the four had a face of a man, but on the right side was the face of a lion, and on the left side the face of an ox, and finally each had the face of an eagle ....”
In the Book of Revelation (4:6-8), we find a similar description:
“Surrounding this throne were 24 other thrones upon which were seated 24 elders; they were clothed in white garments and had crowns of gold on their heads. From the throne came flashes of lightening and peals of thunder; before it burned seven flaming torches, the seven spirits of God. The floor around the throne was like a sea of glass that was crystal-clear. At the very center, around the throne itself, stood four living creatures covered with eyes front and back. The first creature resembled a lion; the second, an ox; the third had the face of a man; while the fourth looked like an eagle in flight. Each of the four living creatures had six wings and eyes all over, inside and out. Day and night, without pause, they sing: ‘Holy, holy, holy, is the Lord God Almighty, He who was, and who is, and who is to come!’”
These images in both the Old Testament and the New Testament prompted St. Irenaeus (140-202) to liken them to the four Gospel writers because of the content of their Gospels and their particular focus on Christ.
Irenaeus came up with the first living creature designations, but the ones from Jerome finally stuck.
Matthew: the winged human to symbolize humanity and reason. The theme of the Gospel is the personhood of Christ and the Gospel begins with the genealogy of Jesus from Abraham through Joseph.
Mark: the winged lion to symbolize royalty, courage, resurrection. The theme of the Gospel is Christ as the Son of God and the Gospel begins with John the Baptist roaring like a lion in the wilderness.
Luke: the winged ox to symbolize sacrifice, strength. The theme of the Gospel is Christ as the healer, priest and sacrifice and the Gospel begins with the temple duties of Zacharias.
John: the eagle to symbolize the heavens, sky, and spirit. The theme of the Gospel is Christ's divine nature and the Gospel begins with Christ as the Eternal Logos, the enduring Word of God.