There was nothing more important in the Jewish religion than the Scriptures. It was sacred and to be followed to the last letter. When John writes his gospel, he intends to show that Jesus was greater than the Jewish Scripture. The way in which he does this is to quote the Old Testament often, and show how Jesus was the promise that the Old Testament spoke of.
Here are some of his quotes.
John 1.23- Is 40.3
The call to make a road in the wilderness for God. This is an announcement that God is coming, and that people should prepare the way for Him to approach.
John 1.51- Gen 28.12
This beautiful promise that one day (and we are living in this time now, because Jesus tore the veil in the Temple that separated the people from God), the relationship between God and man would be restored again. God would walk with us, as He walked with Adam and Eve.
John 2.17- Ps 69:9
The House of the Lord would consume those who believe. We can see so many examples of this in the lives of the Apostles. They were consumed with the gospel message to such an extent that they would die for it. This is because they understood that Jesus too was concerned with the House of His Father. His zeal meant that He was willing to suffer and die on a cross.
John 6.31- Ps 78.23-24
John reminds us that Jesus is the spiritual food from heaven, just like manna was the physical food of the Israelites in the wilderness.
John 7.42- Mic 5.2
Jesus, as prophesied, would come from the least of the tribes of Israel.
John 10.16- Ezek 34.23
Jesus told us himself that He was the Shepard, and that the sheep knows His voice, and follows him.
John 10.34- Ps 82.6
Those who believe are children of the Most High.
John 12.13- Ps 118.26
Jesus, the blessing of all nations, came in the Name of the Lord
John 12.14-15- Zech 9.9
John reminding us of the promise of Jesus’ lowliness.
John 12.31-41 Is 6.10
Jesus came to open the eyes and ears, and soften hardened hearts. John refers to this Scripture to show the state of people before they meet Jesus. I can testify to the fact that this is indeed how I was myself, before I had an encounter with God.
John 13.18- Ps 41.9
John quotes Jesus’ words, which is a direct quote from the Psalms, about the fact that one who was intimate enough to sit at His table and share His bread, would also betray Him.
John 19.36- Exo12.46; Ps 34.20
John repeats the words from Palmist, speaking about the sacrificial Passover Lamb of which the bones were not to be broken. Jesus, as our Sacrificial Lamb, did not have his legs broken on the Cross, as was the custom of the day.
John 19.37- Zech 12.10
The bitterness of those wo saw Jesus, God’s first born, die upon the Cross.