11 All this that the Lord spoke concerning His flesh and blood;— and in the grace of that distribution He promised us eternal life, and that He meant those that eat His flesh and drink His blood to be understood, from the fact of their abiding in Him and He in them; and that they understood not who believed not; and that they were offended through their understanding spiritual things in a carnal sense; and that, while these were offended and perished, the Lord was present for the consolation of the disciples who remained, for proving whom He asked, “Will ye also go away?” that the reply of their steadfastness might be known to us, for He knew that they remained with Him—let all this, then, avail us to this end, most beloved, that we eat not the flesh and blood of Christ merely in the sacrament, as many evil men do, but that we eat and drink to the participation of the Spirit, that we abide as members in the Lord's body, to be quickened by His Spirit, and that we be not offended, even if many do now with us eat and drink the sacraments in a temporal manner, who shall in the end have eternal torments. For at present Christ's body is as it were mixed on the threshing-floor: “But the Lord knows them that are His.” If you know what you thresh, that the substance is there hidden, that the threshing has not consumed what the winnowing has purged; certain are we, brethren, that all of us who are in the Lord's body, and abide in Him, that He also may abide in us, have of necessity to live among evil men in this world even unto the end. I do not say among those evil men who blaspheme Christ; for there are now few found who blaspheme with the tongue, but many who do so by their life. Among those, then, we must necessarily live even unto the end.
Source: Tractates on the Gospel of John (New Advent)