Pilgrim What about what Calvin wrote in his Institutes:
http://www.thevine.net/~phillipj/calvin/bk4ch15.html#sixteen.htm
This confutes the error of the Donatists, who measured the efficacy and worth of the sacrament by the dignity of the minister. Such in the present day are our Catabaptists, who deny that we are duly baptised, because we were baptised in the Papacy by wicked men and idolaters; hence they furiously insist on anabaptism.

Against these absurdities we shall be sufficiently fortified if we reflect that by baptism we were initiated not into the name of any man, but into the name of the Father, and the Son, and the Holy Spirit; and, therefore, that baptism is not of man, but of God, by whomsoever it may have been administered. Be it that those who baptised us were most ignorant of God and all piety, or were despisers, still they did not baptise us into a fellowship with their ignorance or sacrilege, but into the faith of Jesus Christ, because the name which they invoked was not their own but God's, nor did they baptise into any other name. But if baptism was of God, it certainly included in it the promise of forgiveness of sin, mortification of the flesh, quickening of the spirit, and communion with Christ. Thus it did not harm the Jews that they were circumcised by impure and apostate priests. It did not nullify the symbol so as to make it necessary to repeat it. It was enough to return to its genuine origin.