Mark

The lion denotes ST. MARK, because that writer opens his inspired Gospel by describing St. John the Baptist, who was the voice of one crying in the wilderness.

The beginning of the gospel of Jesus Christ, the Son of God. As it is written in Isaiah the prophet, “Behold, I send my messenger before your face, who will prepare your way, the voice of one crying in the wilderness: ‘Prepare the way of the Lord, make his paths straight,’” John appeared, baptizing in the wilderness and proclaiming a baptism of repentance for the forgiveness of sins. And all the country of Judea and all Jerusalem were going out to him and were being baptized by him in the river Jordan, confessing their sins. Now John was clothed with camel's hair and wore a leather belt around his waist and ate locusts and wild honey. And he preached, saying, “After me comes he who is mightier than I, the strap of whose sandals I am not worthy to stoop down and untie. I have baptized you with water, but he will baptize you with the Holy Spirit.” (Mark 1:1-8, ESV).

In the center, around the throne, were four living creatures, and they were covered with eyes, in front and in back. The first living creature was like a lion, the second was like an ox, the third had a face like a man, the fourth was like a flying eagle … Day and night they never stop saying: “Holy, holy, holy is the Lord God Almighty, who was, and is, and is to come.” (Revelation 4:7-8, NIV).