9 For, in the next place, that I may not seem to rest on mere human arguments—since there is so much obscurity in this question, that in earlier ages of the Church, before the schism of Donatus, it has caused men of great weight, and even our fathers, the bishops, whose hearts were full of charity, so to dispute and doubt among themselves, saving always the peace of the Church, that the several statutes of their Councils in their different districts long varied from each other, till at length the most wholesome opinion was established, to the removal of all doubts, by a plenary Council of the whole world: — I therefore bring forward from the gospel clear proofs, by which I propose, with God's help, to prove how rightly and truly in the sight of God it has been determined, that in the case of every schismatic and heretic, the wound which caused his separation should be cured by the medicine of the Church; but that what remained sound in him should rather be recognized with approbation, than wounded by condemnation.
It is indeed true that the Lord says in the gospel, "He that is not with me is against me; and he that gathers not with me scatters abroad." Yet when the disciples had brought word to Him that they had seen one casting out devils in His name, and had forbidden him, because he followed not them, He said, "Forbid him not: for he that is not against us is for us. For there is no man which shall do a miracle in my name, that can lightly speak evil of me." If, indeed, there were nothing in this man requiring correction, then any one would be safe who, setting himself outside the communion of the Church, severing himself from all Christian brotherhood, should gather in Christ's name; and so there would be no truth in this, "He that is not with me is against me; and he that gathers not with me scatters abroad."
But if he required correction in the point where the disciples in their ignorance were anxious to check him, why did our Lord, by saying, "Forbid him not," prevent this check from being given? And how can that be true which He then says, "He that is not against you is for you?" For in this point he was not against, but for them, when he was working miracles of healing in Christ's name. That both, therefore, should be true, as both are true—both the declaration, that "he that is not with me is against me, and he that gathers not with me scatters abroad;" and also the injunction, "Forbid him not; for he that is not against you is for you,"— what must we understand, except that the man was to be confirmed in his veneration for that mighty Name, in respect of which he was not against the Church, but for it; and yet he was to be blamed for separating himself from the Church, whereby his gathering became a scattering; and if it should have so happened that he sought union with the Church, he should not have received what he already possessed, but be made to set right the points wherein he had gone astray?
Source: On Baptism, Against the Donatists (New Advent)