Tertullian, On Exhortation to Chastity 10

We must renounce carnal things so that at some point we may reconsider spiritual ones. Seize the opportunity, even if it is not the one you most desire, for it is still opportune: you no longer have anyone to whom you owe a debt, nor anyone from whom you might be freed; you have ceased to be a debtor. How fortunate you are! You have dismissed the debtor; now endure the loss. But what if what we call loss you experience as gain? For through continence you will indeed secure a great wealth of sanctity; by sparing the flesh, you acquire the spirit. Let us re-examine our own conscience, which a person perceives as different from himself: when he happens to abstain from his own wife, he gains spiritual understanding. If he prays to the Lord, he is near heaven. If he devotes himself to the Scriptures, he is entirely there. If he sings a psalm, it pleases him. If he adjures a demon, he trusts in himself. Therefore the Apostle added a temporary purification for the sake of commendable prayers, so that we may know that what profits for a time must always be exercised by us, so that it may always profit. Prayer is necessary for human beings every day, indeed at every moment, and continence likewise, since prayer is necessary. Prayer proceeds from conscience. If conscience is ashamed, prayer is ashamed. The Spirit leads prayer to God. If the spirit stands guilty before itself because of a conscience that is ashamed, how will it dare to lead prayer from that very source, when even the holy minister is stained by its shame? Indeed, there is a prophetic voice of the Old Testament: "You shall be holy, for I the Lord your God am holy" (Leviticus 11:44); and again: "With the holy you show yourself holy, and with the innocent man you show yourself innocent, and with the elect you show yourself elect" (Psalm 18:26). For we ought to enter the discipline of the Lord in a manner worthy of him, not according to the hot desires of the flesh. For thus the Apostle also says that to set the mind on the flesh is death, but to set the mind on the Spirit is life and peace in Christ Jesus our Lord (Romans 8:6). If this obscuration, even when the fleshly act is exercised within a single marriage, drives away the Holy Spirit, how much more when a second marriage is undertaken?