9 Therefore, if none but He descended and ascended, what hope is there for the rest? The hope for the rest is this, that He came down in order that in Him and with Him they might be one, who should ascend through Him. “He says not, And to seeds,” says the apostle, “as in many; but as in one, And to your seed, which is Christ.” And to believers he says, “And you are Christ's; and if Christ's, then are Abraham's seed.” What he said to be one, that he said that we all are.
Hence, in the Psalms, many sometimes sing, to show that one is made of many; sometimes one sings, to show what is made of many. Therefore was it only one that was healed in the pool; and whoever else went down into it was not healed. Now this one shows forth the oneness of the Church. Woe to them who hate unity, and make to themselves parties among men! Let them hear him who wished to make them one, in one, for one: let them hear him who says, Be not ye making many: “I have planted, Apollos watered; but God gave the increase.
But neither he that plants is anything, neither he that waters; but God that gives the increase.” They were saying, “I am of Paul, I of Apollos, I of Cephas.” And he says, “Is Christ divided?” Be in one, be one thing, be one person: “No man has ascended into heaven, but He who came down from heaven.” Lo! We wish to be yours, they said to Paul. And he said to them, I will not that you be Paul's, but be His whose is Paul together with you.
Source: Tractates on the Gospel of John (New Advent)