Throughout the Bible you would see more than 100 references where Jesus is addressed the Lamb of God. Including by Prophet John when saw Jesus face to face in earth. If you read new testament, you would see that Jesus is very often referred to as "Lamb of God" and "Son of Man".
In fact when Jesus began his ministry work, prophet John the Baptist introduced Jesus to the crowd by saying "The next day John saw Jesus coming toward him and said, "Look, the Lamb of God, who takes away the sin of the world!"[John 1:29]
So why is Jesus called Lamb of God?According to Moses law for the remission of sin one must kill spotless(spiritual meaning blameless) lamb/cow and sprinkle its blood around the alter. "These are the regulations for the guilt offering, which is most holy: The animal sacrificed as a guilt offering must be slaughtered at the place where the burnt offerings are slaughtered, and its blood must be splattered against all sides of the altar" [Leviticus 7:1-2]
For the wages of sin is death and sin is washed by blood of the animal that was sacrificed. Basically spotless animal died as a shubsitue for sinner.
So Jesus who existed even before Abraham(https://www.biblegateway.com/passage/?search=John 8:48-59&version=NLT) came to earth as a Lamb of God to die for your and my sins, so that we don't have to kill anymore animals but believe and claim Jesus died for us as we confess our sins to God the Father. He was sinless(spotless) yet he died a sinner death taking our place. Hence the name Lamb of God.
