And he looked upon Jesus as He walked, and said, “Behold, the Lamb of God.” John 1:36 NAS 

In the Old Testament a lamb was used as an offering/atonement to God for sin. In the New Testament Jesus was the blood offering/atonement for our sin. Through His death the penalty for sin has been paid and the salvation of all men is made possible by His sacrifice. 

“Worthy is the Lamb that was slain to receive power and riches and wisdom and might and honor and glory and blessing.” Revelation 5:12 NAS