Why is Jesus often referred to as the Lamb of God in the Bible? What is the significance of that fact?
Behold the Lamb of God
John the Baptist knew who the Lamb of God was and for what the purpose the Lamb came. When John saw Jesus coming, he announced, “Behold! The Lamb of God who takes away the sin of the world” (John 1:29)! As if to reaffirm this statement, John says on the very next day “And looking at Jesus as He walked, he said, “Behold the Lamb of God” (John 1:35)! John may not have completely understood the ministry of Jesus because John expressed some doubts (Luke 7:20) but he understood enough to know that Jesus was that promised Messiah and that there is no forgiveness without the shedding of blood…and ultimately, the blood of the Lamb of God would be required for the complete forgiveness of sins .
How the Lamb was Slain Before the Earth Existed
When we read about Jesus being called the Lamb of God, there is one reference to Him that many can hardly believe with their own eyes. Revelation 13:8 says that “all that dwell upon the earth shall worship him [the beast], whose names are not written in the book of life of the Lamb slain from the foundation of the world.” Two things we notice here is that for those who are not saved, they are not in the book of life. This book of life is “the book of life of the Lamb.” Jesus has this book and it is His book in which all those who repented and trusted in Him have their names recorded. Also, this Lamb of God “was slain from [or before, literally] the foundation of the world.” Remember that God sees things differently than we do. He sees them as already existing or as already having come to pass. God must have known before the Garden that man would fall…in fact God knew before the earth even existed that a sacrifice was going to be necessary and that is why Jesus is said to “be slain before the foundation of the earth.” Keep in mind this is not the revelation of John but the Book of Revelation is the Revelation of Jesus Christ. Jesus gave John this revelation and it was communicated by an angel of God directly to John (Rev 1:1).
Led to the Slaughter
I have seen lambs being led to the slaughter and even when they are sheared, they are for the most part, silent. Perhaps they are quite because they are afraid. Lambs are extremely timid but when the shearers take hold of them and they are shorn (wool shaven off), they are completely silent. They do not seem to resist. They simply remain still, silent, and submissive to the shearer. See the analogy of Jesus? He was led to the slaughter. He was silent before His accusers. He remained still, trusting that God’s judgment  would come. He was submissive during His scourging and He never resisted, even when they were nailing Him to the cross. So Jesus was “Like a sheep…led to the slaughter and like a lamb before its shearer is silent, so he opens not his mouth” (Acts 8:32) and this Lamb was foretold thousands of years before He came to the earth (Isaiah 53:7).
The Lamb Without Blemish
In the Old Testament the lambs that were to be sacrificed could have no blemishes at all (Ex 12:5) and so this is why Jesus, who was sinless, was called the Lamb of God without blemish. First Peter 1:18-20 says that believers “were redeemed from the empty way of life handed down to you from your ancestors, but with the precious blood of Christ, a lamb without blemish or defect. He was chosen before the creation of the world, but was revealed in these last times for your sake.” Once again Peter mentions that the Lamb of God was “without blemish or defect” and that He “was chosen before the creation of the world.” This was the most perfect Lamb possible and this Lamb, as was mentioned in Revelation 13:8, was chosen before the foundation or creation of the world. This Lamb was chosen for us before we even existed, before the earth was created, and before the fall in the Garden of Eden. This means that God most certainly foreknew that mankind would fall and a Redeemer would be needed. This Lamb must exceed those of the Old Testament for those sacrifices had to be repeated year after year and never really took away the sins because animal blood was not sufficient to clear the guilty and they had to be repeated over and over again, day after day (Heb 10:-4) This spotless Lamb died for the blemished and defective and this sacrifice needed to occur only once and it would be once and for all (Heb 10:12).
You need the Lamb’s blood applied, not to your doorpost as in the Passover (Ex 12:7), but over your life (John 3:16-17). If you have not repented and confessed your sins and expressed your need for a Savior and trusted in Christ for eternal life, you are going to be led to the slaughter someday (Rev 20:11-15). Only the blood of the Lamb of God can take away your sins. I suggest you trust in Christ and have His blood atone for your sins…or you will have to pay for your own sins someday and unfortunately, that will take all of eternity to pay.
Another Reading: Why Does God Test Us? Why Can’t We Test Him? A Bible Study 
Resources: New International Version Bible (NIV) THE HOLY BIBLE, NEW INTERNATIONAL VERSION®, NIV® Copyright © 1973, 1978, 1984, 2011 by Biblica, Inc.™ Used by permission. All rights reserved worldwide