Do Not Assume One Is A Christian Just Because They Claim To Be

As to your saying of us [Christians] that we are a most shameful set, and utterly steeped in luxury, avarice, and depravity, we will not deny that this is true of some. It is, however, a sufficient testimonial for our name, that this cannot be said of all, not even of the greater part of us. It must happen even in the healthiest and purest body, that a mole should grow, or a wart arise on it, or freckles disfigure it. Not even the sky itself is clear with so perfect a serenity as not to be flecked with some filmy cloud.  A slight spot on the face, because it is obvious in so conspicuous a part, only serves to show purity of the entire complexion. The goodness of the larger portion is well attested by the slender flaw. But although you prove that some of our people are evil, you do not hereby prove that they are Christians. Search and see whether there is any sect to which (a partial shortcoming) is imputed as a general stain. You are accustomed in conversation yourselves to say, in disparagement of us, “Why is so-and-so deceitful, when the Christians are so self-denying? why merciless, when they are so merciful?” You thus bear your testimony to the fact that this is not the character of Christians, when you ask, in the way of a retort, how men who are reputed to be Christians can be of such and such a disposition. There is a good deal of difference between an imputation and a name, between an opinion and the truth. For names were appointed for the express purpose of setting their proper limits between mere designation and actual condition. How many indeed are said to be philosophers, who for all that do not fulfill the law of philosophy? All bear the name in respect of their profession; but they hold the designation without the excellence of the profession, and they disgrace the real thing under the shallow pretence of its name. Men are not straightway of such and such a character, because they are said to be so; but when they are not, it is vain to say so of them: they only deceive people who attach reality to a name, when it is its consistency with fact which decides the condition implied in the name. And yet persons of this doubtful stamp do not assemble with us, neither do they belong to our communion: by their delinquency they become yours once more, since we should be unwilling to mix even with them whom your violence and cruelty compelled to recant. Yet we should, of course, be more ready to have included amongst us those who have unwillingly forsaken our discipline than willful apostates. However, you have no right to call them Christians, to whom the Christians themselves deny that name, and who have not learned to deny themselves.

Tertullian, Ad Nationes, Chapter V