To the pious and placid, his advice is mercifully to correct what they can, and to bear patiently with what they cannot correct, in love lamenting and mourning until God either reforms or corrects, or at the harvest root up the tares, and scatter the chaff. Let all the godly study to provide themselves with these weapons, lest, while they deem themselves strenuous and ardent defenders of righteousness, they revolt from the Kingdom of Heaven, which is the only kingdom of righteousness. For as God has been pleased that the communion of His Church shall be maintained in this external society, anyone who, from hatred of the ungodly violates the bond of his society, enters on a downward course, in which he incurs great danger of cutting himself off from the communion of saints. Let them reflect, that in a numerous body there are several who may escape their notice, and yet are truly righteous and innocent in the eyes of the Lord. Let them reflect, that of those who seem diseased, there are many who are far from taking pleasure or flattering themselves in their faults, and who, always aroused by a serious fear of the Lord, aspire to greater integrity. Let them reflect, that they have no right to pass judgment on a man for one act, since the holiest sometimes make the most grievous fall. Let them reflect, that in the ministry of the Word and participation of the sacraments, the power to collect the Church is too great to be deprived of all its efficacy, by the fault of some ungodly men. Lastly, let them reflect, that in estimating the Church, divine is of more force than human judgment. 

   Since they also argue that there is good reason for the Church being called holy, it is necessary to consider what the holiness is in which it excels, lest by refusing to acknowledge any church, save one that is perfect, we leave no church at all. It is indeed true, as Paul says, that Christ “loved the church, and gave himself for it, that he might sanctify and cleanse it with the washing of water by the word, that he might present it to himself a glorious church, not having spot, or wrinkle, or any such thing; but that it should be holy and without blemish” (Ephesians 5:25-27). Nevertheless, it is true, that the Lord is daily smoothing its wrinkles, and wiping away its spots. Hence, it follows, that its holiness is not yet perfect. Such, then, is the holiness of the Church: it makes daily progress, but is not yet perfect; it daily advances, but as yet has not reached the goal, as will elsewhere be more fully explained.

The Holy Catholic Church (Book 4 of the Institutes) - John Calvin