Early Church Writers
Irenaeus, (born in 130 AD) said God revealed Himself through prophets’ deeds. Hosea’s taking a wife of whoredom foreshadowed Christ. It showed “it will be God’s good pleasure to take out a church which shall be sanctified by fellowship with His Son, just as that woman was sanctified by intercourse with the prophet. And for this reason, Paul declares that the unbelieving wife is sanctified by the believing husband.” He later talked about Moses taking an Ethiopian wife. This showed the wild olive tree was grafted into the cultivated one.
Both Moses and Hosea knew God before marriage. Neither example showed two unbelievers marrying and one converting. If Irenaeus thought 1 Corinthians 7:14 only applied to existing unions, why didn’t he mention it? Irenaeus probably believed the time of conversion didn’t play into it.
Origen (born 185) claims Paul gave additional charges to the church, like Moses and the bill of divorce. 1 Corinthians 7:12-14 was an example of such a command because Paul said he had no law from the Lord.
Although Paul said the believer would sanctify the unbeliever, Origen said he could be wrong. The reverse could happen; the unbeliever could profane, or lead astray, the believer. Origen says a person should therefore consider both present and future. After examining these, they should either not marry or marry in the Lord. “We do not obey the verse only in the Lord [if we marry a non-believer], but when we read that she is free to be married to whomever she wishes, we have not yet connected it with only in the Lord.” Earlier, he advocated God’s laws as better than Paul’s.
Shaye Cohen believes Origen understood the verses as an allowance to contract a mixed marriage, not merely to continue an existing one.
Origen’s commentary on Matthew may clarify. It too mentioned Moses allowing divorce for people’s hard hearts. He mentioned church leaders letting a woman remarry despite Romans 7:3. Origen said, “[T]his concession was permitted in comparison with worse things.” Romans 7:3 forbade a woman from marrying another while her husband lived. Likewise, to Origen, 1 Corinthians 7:39 forbade mixed marriages. In both cases, church leaders gave different rulings for the hardness of people’s hearts.
Tertullian (born 160) first proposed limiting the verse to pre-existing unions. Even in his works, Caroline points out, “We have evidence of ‘unpublished Christians’- those whose opinions are not represented by their own extant texts, but whose views might be recovered in the published arguments of others-who read Paul differently.” One proposed example includes women and counselors seeing 1 Cor. 7:14 as approval to marry an outsider. In Caroline’s terminology, Tertullian alluded to advisors who “supported their decision and interpretation of Paul.” He recorded a man saying marrying an unbeliever was a tiny sin. Caroline closes by saying the unpublished and published Christian writers “signal that boundaries were not always clear between Christians and non-believers.”
Later Church Writers
However, a parade of Christian writers born in the 300s argued Paul addressed only the already married and/or didn’t give the excuse to marry an unbeliever. This includes Pelagius (born 360 AD), Theodoret (born 393 AD), Ambrosiaster (fl. C 366-384), and Severian of Gabala (died after 408).
Augustine (born 354) mentioned in Ezra in the Old Testament where the Israelites put away their unbelieving wives. In contrast, he disagreed with commanding a believer in the New Testament to divorce an unbeliever. When they married, both were gentiles. They did nothing the Lord forbade when they married.
John Chrysostom (born 347) said, “For the question now is not about those who have never yet come together, but about those who are already joined. He did not say, If anyone wishes to take an unbelieving wife, but If anyone has an unbelieving wife. Which means if any after marrying or being married have received the word of godliness, and then the other party which had continued in unbelief still yearn for them to dwell together, let not the marriage be broken off.”
Jerome (born 347) said the couple married, and one got saved. “In the above discussion, the Apostle has taught that the believer ought not to depart from the unbeliever, but remain in marriage as the faith found them and that each man, whether married or single, should continue as he was when baptized into Christ... Whether he had, or had not, a wife when he believed, let him remain in that condition in which he was when called.”
Perhaps the earliest Christians didn’t limit the verses. We can’t be sure.
Irenaeus’ reference to Hosea argues from silence at best. Tertullian suggested some may have considered it acceptable to marry an unbeliever due to 1 Corinthians 7:12-14, but were there such advisors? He didn't explicitly say. The church agreed in the 300s that the verse only applied to existing marriages.