Jesus Christ is the fullness of God’s revelation in the sense that through Him God has communicated to humanity the totality of truths He chose to reveal to man though at the same time the Church admits that Christian revelation is not yet completely explicit. Through Christ it was God Himself who makes known to man His will.
In the Old Testament, God has spoken through prophets. They were God’s tools in communicating His laws and decrees to Israel. For a period it was this instrumentality that bridges God’s revelation. But God has to choose prophets every now and then since as human beings they die and because of human frailties they have the tendency to fall back as evidenced by God’s numerous admonitions towards Israel and her prophets. The mystery of the Incarnation then was the breakthrough of God’s love towards humanity, an immense generosity on God’s part to send His Son to ferry unto mankind the revelation of the Triune God.
The following is a further presentation of this thesis.
Being fully human, it would seem that Christ’s knowledge could be limited to human finiteness. His person then would be conditioned by his humanity. And if he has limited knowledge then he could only reveal limited truths. This argument would seem to counter the truth that Christ is the fullness of revelation if taken at surface value. However, it would do well to point out that the Church not only believe that Jesus Christ is 100% human but that he is also 100% divine and that His divine nature is inseparable to Him even during His incarnation.
“ In the beginning was the Word, and the Word was with God and the Word was God.” John 1:1
So Jesus is the Word Incarnate, the divinity who took for Himself our humanity. Being fully divine, He has all the means to reveal to its fullness the revelation of God. Further, even in His humanity Jesus was the perfect representation of who God is.
“The Son is the image of the invisible God…” Colossians 1;15
Being in the image of God and the Incarnate God, Jesus indeed possesses the entire truths of God and through Christ God has said everything . (CCC 65)
“In many and various ways God spoke of old to our fathers by the prophets, but in these last days he has spoken to us by a Son.” Hebrews 1:1-2
God having spoken to us everything through His Son is evidenced by the fact that Jesus Himself is the Word. He is the definitive Word coming from God Himself and not only is He the Word He is the unsurpassable Word and no other Word will come after Him. As what St. John of the Cross said in his comment on Hebrews 1:1-2, “…he has spoke everything to us at once in this sole Word – and he has no more to say…” (CCC 65)
Yes, He has no more to say and that means there could be no new public revelation after our Lord Jesus Christ. He has fulfilled God’s Word – He is the Word and we the subject of His revelation ought to follow that WORD.