For there must be also heresies among you
&c.] This is a reason why he was ready to believe there might be something of truth in the report he had received of the divisions among them; for if there were heresies, false doctrines, and bad principles, among them, such as were subversive of the fundamentals of Christianity, as the denial of the resurrection of the dead it was no wonder that there were schisms and factions among them, since heresies generally issue in them. These, the apostle says, "must be"; because God has decreed they shall, whose counsel is immutable, and his purpose unalterable; and since this always was the case, that there were false prophets under the former dispensation, it must be expected that false teachers will arise in the churches now, bringing in damnable heresies; and since Satan is always busy to sow the tares of false doctrine; and human nature, being both weak and wicked, is so susceptible thereof, and so easily imposed upon and deceived, it cannot be thought that it should be otherwise; which, by the goodness and wisdom of God, are overruled to a very good purpose:
that they which are approved:
who sincerely believe in Christ, are sound in the faith, and have a well grounded experience of it; who have themselves tried things that differ, and approve of them that are excellent, and have been tried by others, and found to be sincere, upright, and faithful, and are approved of God and good men:
may be made manifest among you;
by their steadfastness in the faith, their zealous attachment to it, earnest contention for it, and warm and honest vindication of it; and by the departure of those from them who oppose it, and go on the side of error and heresy; by which means it is known who are the sincere followers of the Lamb, in doctrine, discipline, and conversation, and who not.