Matthew Poole and continuators on 1 Corinthians 7:39

39 The wife is bound by the law as long as her husband liveth: but if her husband be dead, she is at liberty to be married to whom she will , only in the Lord .

The Apostle all along this Chapter hath been speaking to several cases, which the Church of Corinth had put to him concerning marriage: some that concerned persons already married: others that concerned such as were single, having been never married, he shutteth up his discourse with advice, which relateth to such as had lost their husbands, with reference to second marriages. As to this he determineth, That no woman might marry again while her first husband lived, that is, unless that her husbands be legally divorced from her for Adultery, or unless her husband, being an Heathen, had voluntarily deserted her: but if her husband were dead, she might marry to whom she would;

yet she was not at such liberty, as that she might marry an Vnbeliever. Unbelievers are either Heathens, or Christians in name; but such as are Idolaters, or profane persons, or Hereticks, who hold such Tenets as are inconsistent with any true Faith in Jesus Christ. This phrase, only in the Lord, seemeth to oblige godly women, not only to avoid marrying with Heathens, but with nominal Christians, that is, such who, although they have been baptized, and owne Christ with their tongues, yet hold such damnable Opinions, or live such profane lives, or worship God in such an idolatrous manner, as is inconsistent with any true Faith in Christ. The reason of the precept holds as well to the latter, as to the former.

Marginal references: Rom. 7.2.