The Gospel of John is rich in theology. I’m glad that it was assigned last, in terms of Gospel readings, in the Nativity Fast New Testament Challenge. Over the years two particular passages have been helpful in my spiritual journey. One is John chapter 6. In John 6, Jesus teaches explicitly that unless you eat His flesh and drink His blood that you have no life in you:
I am the bread of life. Your ancestors ate the manna in the desert, but they died; this is the bread that comes down from heaven so that one may eat it and not die. I am the living bread that came down from heaven; whoever eats this bread will live forever; and the bread that I will give is my flesh for the life of the world.” The Jews quarreled among themselves, saying, “How can this man give us [his] flesh to eat?”
Jesus said to them, “Amen, amen, I say to you, unless you eat the flesh of the Son of Man and drink his blood, you do not have life within you. Whoever eats my flesh and drinks my blood has eternal life, and I will raise him on the last day. For my flesh is true food, and my blood is true drink. Whoever eats my flesh and drinks my blood remains in me and I in him. Just as the living Father sent me and I have life because of the Father, so also the one who feeds on me will have life because of me. This is the bread that came down from heaven. Unlike your ancestors who ate and still died, whoever eats this bread will live forever.” John 6: 48-58
A clearer teaching on the real presence in the Eucharist is not to be found, in my opinion, in the New Testament.
The other passage is Jesus’ prayer for the unity of the Church. My father asked me today, “Why are you Catholic and not Orthodox?” I think this prayer is one of the reasons for my remaining a Greek Catholic. The prayer is found in John: 17:20-23:
I pray not only for them, but also for those who will believe in me through their word, so that they may all be one, as you, Father, are in me and I in you, that they also may be in us, that the world may believe that you sent me.And I have given them the glory you gave me, so that they may be one, as we are one, I in them and you in me, that they may be brought to perfection as one, that the world may know that you sent me, and that you loved them even as you loved me.
All four Gospels down, the Catholic Epistles, and the Epistles of Paul as well. All that remains is the Book of Revelation in the New Testament Nativity Fast Challenge.