The Sufficiency of the Christ
The second half of John 3 presents an interesting “competition” between John the Baptist and Jesus. At least from the perspective of the disciples of John who raised the issue that “all are going to [Jesus]” (John 3:26), the baptismal competition between John and Jesus was threatening John’s significance. But the Baptist’s response to his disciples sets things straight. There is no threat to his significance because Jesus is the Christ while John has always been the forerunner to the Christ, and only that. His response is a good word for us. We are not the Christ, and we will be saved much heartache and headache when we live the Baptist’s attitude.
Then, John the gospel writer finishes out chapter 3 reflecting on why we are not the Christ, but Jesus is the Christ. Jesus is the Messiah, the one who must increase while we all must diminish, because his origin, relations, and gifts are all greater than what we could ever muster.
Indeed, part of the point of John’s reflection at the end of John 3 is to hold up the sufficiency of Jesus as the Messiah. That is to say, Jesus needs no enhancement or improvement from you or from me. He is, on his own, sufficient to complete the will of his Father in heaven (cf. John 4:34). And when we think about the sufficiency of Jesus Christ, we have, for good reason, his sacrificial death on our minds. As the writer to the Hebrews puts it, “But when Christ had offered for all time a single sacrifice for sins, he sat down at the right hand of God, 13 waiting from that time until his enemies should be made a footstool for his feet. 14 For by a single offering he has perfected for all time those who are being sanctified. (Heb 10:12 ESV).
From this text in Hebrews we are reminded that Christ’s sacrifice is perfect in the sense that past, present, and future sins have been atoned for by his single sacrifice. There is no longer any need for a sacrifice, bloody or bloodless, because his one sacrifice of himself is sufficient to cover all the sins for which he died. Moreover, his one sacrifice is sufficient to cover all the sins of all the sinners whom God has chosen before the foundation of the world in him (cf. Eph 1:4). Hypothetically, even if Christ died with the intent of providing potential salvation for anybody and everybody (which he didn’t), his sacrifice would still be sufficient to cover the sins of every single man, woman, and child ever born on this earth, Hitler, Stalin, etc. included. Such is the sufficiency of Christ in his sacrifice. His substitution is entirely sufficient; no enhancement or improvement by us is necessary. As Augustus Toplady put it, “Nothing in my hand I bring, simply to thy cross I cling.”
But Christ’s substitutionary self-sacrifice is not the extent of his sufficiency. His passive obedience, being made sin for us (cf. 2 Cor 5:21) and taking for himself the penalty for all of our sins, is only half of the equation. We also receive his perfect righteousness, his active obedience to the will of his Father in heaven, and this highlights the depths of the sufficiency of Jesus the Messiah, the savior of the world. Douglas Kelly draws out the significance of Christ’s active obedience:
In this active obedience of sanctification and reconciliation, Christ lives out the life of filial sonship which neither Adam nor Israel ever did. Hence, in addition to the biblical concept of substitution, one must add the biblical concept of representation. That is what we find in the theology of the Apostle Paul, especially in Romans 5 and 6, which explicate in detail the union of believers with Christ, their covenant head and representative. This means that as our substitute and representative, as the Last Adam, Christ turns back our humanity from saying with Adam, “My will be done,” (as he succumbed to the temptation of Satan to be as God) to saying, “Thy will be done.” Christ represents us in his true and full humanity by restoring us to God-centeredness.
This God-centeredness rather than me-centeredness includes mind, heart, and will, so that it is a perfect obedience both internally and externally. Kelly again says it well: “We can think of Christ's internal (not merely external) fulfilling of the law and will of God as a sanctification from the inside out of our humanity that was turned away from God. He turns it back around, face to face with the heavenly Father.”
And so, in both his passive and active obedience, the Lord Jesus Christ is all sufficient. Therefore, he must increase, and we must decrease.
 Douglas F. Kelly, Systematic Theology: The Beauty of Christ, A Trinitarian Vision (Fearn: Christian Focus, 2014), 2:334.  Kelly, Systematic Theology, 2:333.