1 Corinthians 7 provides another strong piece of evidence in favor of the children of believers belonging to the people of God. This passage states:
1 Corinthians 7:12-14 12 But to the rest I say, not the Lord, that if any brother has a wife who is an unbeliever, and she consents to live with him, he must not divorce her. 13 And a woman who has an unbelieving husband, and he consents to live with her, she must not send her husband away. 14 For the unbelieving husband is sanctified through his wife, and the unbelieving wife is sanctified through her believing husband; for otherwise your children are unclean, but now they are holy.
In the context of the unity of the family in the eyes of God, the belief of one spouse brings about the covenantal sanctification or holiness of the other spouse. In other words, the union as a whole is undefiled. As evidence of the spousal sanctification, Paul states that the children of the mixed marriage are not in fact unclean (defiled), but are holy. The unbelieving spouse as well as the covenant children have been set apart and claimed by God. If there was no covenantal sanctification of the spouse, then the children would also be unclean.
John Calvin’s commentary on this passage is excellent, and does a better job of explaining it than I can:
It is an argument taken from the effect — “If your marriage were impure, then the children that are the fruit of it would be impure; but they are holy; hence the marriage also is holy. As, then, the ungodliness of one of the parents does not hinder the children that are born from being holy, so neither does it hinder the marriage from being pure.” Some grammarians explain this passage as referring to a civil sanctity, in respect of the children being reckoned legitimate, but in this respect the condition of unbelievers is in no degree worse. That exposition, therefore, cannot stand. Besides, it is certain that Paul designed here to remove scruples of conscience, lest any one should think (as I have said) that he had contracted defilement. The passage, then, is a remarkable one, and drawn from the depths of theology; for it teaches, that the children of the pious are set apart from others by a sort of exclusive privilege, so as to be reckoned holy in the Church.
But how will this statement correspond with what he teaches elsewhere — that we are all by nature children of wrath; (Ephesians 2:3) or with the statement of David — Behold I was conceived in sin, etc. (Psalms 51:5) I answer, that there is a universal propagation of sin and damnation throughout the seed of Adam, and all, therefore, to a man, are included in this curse, whether they are the offspring of believers or of the ungodly; for it is not as regenerated by the Spirit, that believers beget children after the flesh. The natural condition, therefore, of all is alike, so that they are liable equally to sin and to eternal death. As to the Apostle’s assigning here a peculiar privilege to the children of believers, this flows from the blessing of the covenant, by the intervention of which the curse of nature is removed; and those who were by nature unholy are consecrated to God by grace. Hence Paul argues, in his Epistle to the Romans, (Romans 11:16) that the whole of Abraham’s posterity are holy, because God had made a covenant of life with him — If the root be holy, says he, then the branches are holy also. And God calls all that were descended from Israel his sons’ now that the partition is broken down, the same covenant of salvation that was entered into with the seed of Abraham is communicated to us. But if the children of believers are exempted from the common lot of mankind, so as to be set apart to the Lord, why should we keep them back from the sign? If the Lord admits them into the Church by his word, why should we refuse them the sign?
While marriage to an unbeliever is forbidden to a Christian, situations often arise in which conversion (or repentance after a sinful choice to marry) leads to a faithful Christian being married to a non-believer. Paul says in these cases, the believer should stay with the unbeliever, sanctifying the whole household and in particular making the children holy. And, as Calvin reasons, if the children of even a mixed-marriage are holy or set apart covenantally to God, then why should covenant children be denied the sign of the covenant (baptism) and be included in the covenant community?
This New Covenant instruction concerning mixed marriages is a fascinating reversal of Old Covenant instructions for a similar situation, found in Ezra 10. In this case, the exiles of Judah and Benjamin who had returned to the land were coming to repentance over the fact that they had married many foreign wives (i.e. worshippers of false gods). Ezra (a faithful priest and descendant of Aaron) requires the Israelites to divorce these women, and even send the children of the union away from Israel (Ezra 10:3). The instructions of Paul are consistent with an overall change in the flow of sanctification/uncleanliness (defilement) with the coming of Christ. That which the believer previously had to be separated and isolated from (an unbelieving spouse) can now be lived with peacefully, having the expectation that rather than the impurity of the unbeliever contaminating the believer, the purity and good behavior of the believing spouse (see also 1 Peter 3:1-2) might win over the unbeliever.