To quote from Richard Taylor’s, “How To Read a Church“, one of my most thumbed through reference books, “St. Matthew, St. Mark, St. Luke and St. John, each a writer of the four Gospels, are known as the Four Evangelists. Their symbols are, respectively, a man (or angle), a lion (often with wings), a bull or ox (likewise), and an eagle“.
All four symbols are used by the Prophet Ezekiel in the Old Testament, “As for the likeness of their faces, they four had the face of a man, and the face of a lion, on the right side: and they four had the face of an ox on the left side; they four also had the face of an eagle” (Ezekiel 1:10) and The Book of Revelation, “ And the first beast was like a lion, and the second beast like a calf, and the third beast had a face as a man, and the fourth beast was like a flying eagle” (Revelation 4:7).
To quote from Taylor again, “St. John was ascribed the eagle because his Gospel is the most soaring and revelatory, and the eagle in mythology is the only bird able to look directly into the light of the sun“.