91 I have said nothing yet of the internal warfare within ourselves, and in our passions, in which we are engaged night and day against the body of our humiliation, either secretly or openly, and against the tide which tosses and whirls us hither and there, by the aid of our senses and other sources of the pleasures of this life; and against the miry clay in which we have been fixed; and against the law of sin, which wars against the law of the spirit, and strives to destroy the royal image in us, and all the divine emanation which has been bestowed upon us; so that it is difficult for anyone, either by a long course of philosophic training, and gradual separation of the noble and enlightened part of the soul from that which is debased and yoked with darkness, or by the mercy of God, or by both together, and by a constant practice of looking upward, to overcome the depressing power of matter.
And before a man has, as far as possible, gained this superiority, and sufficiently purified his mind, and far surpassed his fellows in nearness to God, I do not think it safe for him to be entrusted with the rule over souls, or the office of mediator (for such, I take it, a priest is) between God and man.
Source: Orations (New Advent)