12 But what follows? “For the poor you have always with you, but me ye will not have always.” We can certainly understand, “the poor you have always;” what He has thus said is true. When were the poor wanting in the Church? “But me ye will not have always;” what does He mean by this? How are we to understand, “Me ye will not have always”? Don't be alarmed: it was addressed to Judas. Why, then, did He not say, you will have, but, ye will have? Because Judas is not here a unit.
One wicked man represents the whole body of the wicked; in the same way as Peter, the whole body of the good, yea, the body of the Church, but in respect to the good. For if in Peter's case there were no sacramental symbol of the Church, the Lord would not have said to him, “I will give unto you the keys of the kingdom of heaven: whatsoever you shall loose on earth shall be loosed in heaven; and whatsoever you shall bind on earth shall be bound in heaven.” If this was said only to Peter, it gives no ground of action to the Church.
But if such is the case also in the Church, that what is bound on earth is bound in heaven, and what is loosed on earth is loosed in heaven,— for when the Church excommunicates, the excommunicated person is bound in heaven; when one is reconciled by the Church, the person so reconciled is loosed in heaven:— if such, then, is the case in the Church, Peter, in receiving the keys, represented the holy Church. If, then, in the person of Peter were represented the good in the Church, and in Judas' person were represented the bad in the Church, then to these latter was it said, “But me ye will not have always.”
But what means the “not always;” and what, the “always”? If you are good, if you belong to the body represented by Peter, you have Christ both now and hereafter: now by faith, by sign, by the sacrament of baptism, by the bread and wine of the altar. You have Christ now, but you will have Him always; for when you have gone hence, you will come to Him who said to the robber, “Today shall you be with me in paradise.” But if you live wickedly, you may seem to have Christ now, because you enter the Church, signest yourself with the sign of Christ, art baptized with the baptism of Christ, minglest yourself with the members of Christ, and approachest His altar: now you have Christ, but by living wickedly you will not have Him always.
Source: Tractates on the Gospel of John (New Advent)