1 Opportunely has the Lord procured for us that this passage should occur in its order to day: for I suppose you have observed, beloved, that we have undertaken to consider and explain the Gospel according to John in due course. Opportunely then it occurs, that today you should hear from the Gospel, that, “Unless a man be born again of water and of the Spirit, he shall not see the kingdom of God.” For it is time that we exhort you, who are still catechumens, who have believed in Christ in such wise, that you are still bearing your sins.
And none shall see the kingdom of heaven while burdened with sins; for none shall reign with Christ, but he to whom they have been forgiven: but forgiven they cannot be, but to him who is born again of water and of the Holy Spirit. But let us observe all the words what they imply, that here the sluggish may find with what earnestness they must haste to put off their burden. For were they bearing some heavy load, either of stone, or of wood, or even of some gain; if they were carrying grain, or wine, or money, they would run to put off their loads: they are carrying a burden of sins, and yet are sluggish to run. You must run to put off this burden; it weighs you down, it drowns you.
Source: Tractates on the Gospel of John (New Advent)