Tertullian

“ Chapter III.—Weak People Fall an Easy Prey to Heresy, Which Derives Strength from the General Frailty of Mankind. Eminent Men Have Fallen from Faith; Saul, David, Solomon. The Constancy of Christ.
It is usual, indeed, with persons of a weaker character, to be so built up (in confidence) by certain individuals who are caught by heresy, as to topple over into ruin themselves. ”

“ But what if a bishop, if a deacon, if a widow, if a virgin, if a doctor, if even a martyr, [8] have fallen from the rule (of faith) , will heresies on that account appear to possess [9] the truth? Do we prove the faith [10] by the persons, or the persons by the faith? No one is wise, no one is faithful, no one excels in dignity, [11] but the Christian; and no one is a Christian but he who perseveres even to the end. [12] You, as a man, know any other man from the outside appearance. You think as you see. And you see as far only as you have eyes. ”