There are several questions here to be answered:
- Are there special rules for a widower as opposed to someone who has never been married?
- In what circumstances is it sinful to remarry after divorce?
- Is it ever a "mortal sin" to remarry after divorce?
I will answer these questions using publications of the Assemblies of God, the largest Pentecostal denomination, and the writings of prominent Renewal theologian J. Rodman Williams. While there is likely some variation in pentecostal beliefs among the many dozens of small denominations, I have found no explicit disagreement between these two major expounders of Pentecostal/Charismatic thought.
Remarriage after being widowed
Neither the AG nor Williams explicitly cover the question of widows remarrying in their treatments of remarriage, which isn't surprising given that very few Christians believe that one who is widowed ought not remarry. However, the AG tangentially addresses the question of remarriage of the widowed in several places, in particular when analyzing the qualifications for church officers (1 Timothy 3). It rejects the idea that the literal rendering, "one woman man," refers to a prohibition of remarriage after being widowed. Thus, even widowed church officers, being held to a standard at least as high as others, may remarry.1
Remarriage after divorce
Remarriage among those who have been divorced, however, is a more complex question. Williams writes:
It is a tragic thing to see two people, having become "one flesh," trying to live separate lives because of divorce; but it is an evil thing to see either or both of them trying to contract another marriage. It is evil, indeed perverted, because the original marriage had made two people one.2
However, he makes an exception in the case of one who was the victim of adultery:
It may be asked, what if, in the marriage, only one partner proves to be unfaithful. Are there not Scriptural grounds for a legitimate re-marriage of the innocent party? The answer to that question is "Yes" [...] Re-marriage and not committing adultery is possible if the divorce resulted from "unchastity." The one who has remained chaste may re-marry without committing adultery.2
The same view is taken by the Assemblies of God:
One who has been divorced because of the repeated adultery of a partner is not bound by the former marriage and is free to remarry.3
Jesus clearly assumed that those who were divorced by sinful spouses, or those who divorced sinful spouses for “marital uncleanness” or abandonment, were free to remarry without any tinge of adultery.1
The term mortal sin, often associated with Roman Catholicism and contrasted with venial sin, appears in some translations of 1 John 5. In Protestantism generally, and Pentecostalism particularly, this sin is often seen to be the same as the sin Jesus called "unforgivable" or "unpardonable" in Matthew 12:31 and elsewhere. It is only with this understanding that we can make any sense of the question of "is divorce after remarriage a mortal sin," since Pentecostalism does not classify sin in the same way that Roman Catholicism does.
Turning now to Williams, we find that he does believe that Christians can commit a "mortal" or "unforgivable" sin, which will not be forgiven, he does not say that about remarriage after divorce:
if we have been divorced and re-married, but not on the grounds of the Scripture [...] if we are truly sorry, God, for Christ's sake, will always forgive.2
Rather, the unpardonable sin, quoting the AG, is "more descriptive of a spiritual state or condition than a single act of sin." It "involves a deliberate or willful act of unbelief," and is not "committed in ignorance" nor is it a "single momentary act."4
By failing to abide in Christ, to continue in Him and His word, to persevere in the midst of worldly trial or temptation, to make faith firm and strengthen it—thereby allowing unbelief to enter—believers may fall away from God. Thereby they may tragically forfeit their salvation.5
Can those who remarry after divorce commit a "mortal" ("unpardonable") sin, according to Pentecostalism? Perhaps, yes, if they do so willfully and as part of a pattern of unbelief. However, grace is freely offered to those who truly repent of their sin.
Though remarriage after divorce is generally sinful, exceptions are made for adultery or abandonment: in such cases, the innocent victim may remarry without sinning.
- AG, "Divorce and Remarriage" position paper
- Williams, The Ten Commandments, "The Seventh Commandment". Emphasis in original.
- AG, "Divorce and Remarriage" summary
- AG, "Unpardonable sin"
- Williams, Renewal Theology, Vol. II, 129