When the apostle Paul writes to the church of Corinth about marriage, he makes a confusing statement that an unbelieving spouse is sanctified by the believing spouse:

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. And a woman who has an unbelieving husband, and he consents to live with her, she must not send her husband away. 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. Yet if the unbelieving one leaves, let him leave; the brother or the sister is not under bondage in such cases, but God has called us to peace. For how do you know, O wife, whether you will save your husband? Or how do you know, O husband, whether you will save your wife? (1 Cor 7:12-16)

Adding to this confusion, Paul indicates that the believing spouse is also the reason that their children are clean and considered holy! A valid question is elicited: isn't faith required to make one sanctified and holy?

As a port, Corinth was a wealthy city that had the largest population of Greeks, Jews, and Romans in Greece. While the majority of Corinth was Greek, the Latin names mentioned in Paul's letter attest to its Roman influence; some Jews had Roman names like Aquila (1 Cor 16:19) and Crispus (1 Cor 1:14). In addition to wealth, the culture of Corinth was influenced by several cults that glorified pagan knowledge and wisdom and encouraged sexual immorality as a worship practice.

Against this pagan religious background of culturally acceptable practices of ecstatic utterances, wisdom, law suits, and promiscuity, the church of Corinth was divided (1 Cor 1:11) and the apostle Paul was informed of some controversies and conflicts in teaching about a variety of topics including marriage and relationships involving sex.

Paul devotes some time on the issue of promiscuity and its danger to the church; immorality within the church is akin to yeast that permeates the dough (1 Cor 5:6-13). On a personal level, to continue with a sexually immoral life is to make your body a member of a prostitute rather that a member of Jesus Christ:

Food is for the stomach and the stomach is for food, but God will do away with both of them. Yet the body is not for immorality, but for the Lord, and the Lord is for the body. Now God has not only raised the Lord, but will also raise us up through His power. Do you not know that your bodies are members of Christ? Shall I then take away the members of Christ and make them members of a prostitute? May it never be! Or do you not know that the one who joins himself to a prostitute is one body with her? For He says, "The two shall become one flesh." But the one who joins himself to the Lord is one spirit with Him. Flee immorality. Every other sin that a man commits is outside the body, but the immoral man sins against his own body. Or do you not know that your body is a temple of the Holy Spirit who is in you, whom you have from God, and that you are not your own? For you have been bought with a price: therefore glorify God in your body. (1 Cor 6:13-20)

In establishing the biblical view of the human body, Paul reminds Believers of their sanctity as the temple of the Holy Spirit; he repeats to emphasize a comment he made earlier in his letter:

"Do you not know that you are a temple of God and that the Spirit of God dwells in you? If any man destroys the temple of God, God will destroy him, for the temple of God is holy, and that is what you are." (1 Cor 3:16-17)

Following his message about the sanctity of the body, mixed marriages, those between Believers and non-Believers, are of particular concern to the church of Corinth, and it is here that Paul introduces the idea that a believing spouse sanctifies the non-believing spouse and their children.

The concept of "to sanctify" finds its basis in the Old Testament. It is all of those things that God separates and devotes to Himself that become holy: time (Gen 2:3), space (Ex 3:4-5), objects and people (Ex 13:2); God essentially separates and devotes to Himself time and three dimensional space. In the New Testament, it is in Jesus, joined to Him and in one Spirit with Him (1 Cor 6:17) that a Believer is sanctified and made holy (Acts 26:15-18).

Because woman is organically created from man, God indicates that marriage is figuratively representative of that physical origin, which He sees as one flesh.

It is in the context of being of one flesh with a Believer that sanctifies the non-believing spouse. And because of the Believer's faith, his / her children are also sanctified – set apart.

Peter said to them, "Repent, and each of you be baptized in the name of Jesus Christ for the forgiveness of your sins; and you will receive the gift of the Holy Spirit. For the promise is for you and your children and for all who are far off, as many as the Lord our God will call to Himself." (Acts 2:38-29)

Based on His earliest commands in the Old Testament, God determines what is set apart on earth as seen in this example:

'You are therefore to keep all My statutes and all My ordinances and do them, so that the land to which I am bringing you to live will not spew you out. Moreover, you shall not follow the customs of the nation which I will drive out before you, for they did all these things, and therefore I have abhorred them. Hence I have said to you, "You are to possess their land, and I Myself will give it to you to possess it, a land flowing with milk and honey." I am the Lord your God, who has separated you from the peoples. You are therefore to make a distinction between the clean animal and the unclean, and between the unclean bird and the clean; and you shall not make yourselves detestable by animal or by bird or by anything that creeps on the ground, which I have separated for you as unclean. Thus you are to be holy to Me, for I the Lord am holy; and I have set you apart from the peoples to be Mine. (Lev 20:22-26)

Note carefully that the promise and inheritance of land is conditional; it requires that the nation of Israel to have the faith and obedience to keep all of God's statues and ordinances.

While the non-Believer is sanctified through the believing spouse, because they are of "one flesh," it does not mean that the non-Believer is saved with eternal life - joining in flesh physically is not the same as joining to the Lord in spirit.

For He says, "The two shall become one flesh." But the one who joins himself to the Lord is one spirit with Him. (1 Cor 6:16b-17)

Circumcision is nothing, and uncircumcision is nothing, but what matters is the keeping of the commandments of God. (1 Cor 7:19)

Paul makes clear that it is faith in Jesus that a Believer is eternally sanctified and made holy. It is the indwelling of the Holy Spirit that guarantees one's entrance into the kingdom of God and inheritance as God's adopted (Eph 1:13-14; 2 Cor 1:21-22; 5:5).

The apostle Peter makes the connection that through Jesus Christ, one becomes a person of God's own possession among and a part of God's kingdom of priests and holy nation.

And coming to Him as to a living stone which has been rejected by men, but is choice and precious in the sight of God, you also, as living stones, are being built up as a spiritual house for a holy priesthood, to offer up spiritual sacrifices acceptable to God through Jesus Christ…. But you are a chosen race, a royal priesthood, a holy nation, a people for God's own possession, so that you may proclaim the excellencies of Him who has called you out of darkness into His marvelous light; for you once were not a people, but now you are the people of God; you had not received mercy, but now you have received mercy. (1 Pet 2:4-10)

Now then, if you will indeed obey My voice and keep My covenant, then you shall be My own possession among all the peoples, for all the earth is Mine; and you shall be to Me a kingdom of priests and a holy nation.' These are the words that you shall speak to the sons of Israel." (Ex 19:5-6)

Jesus alludes to this in His upper room discourse.

If you had known Me, you would have known My Father also; from now on you know Him, and have seen Him." Philip said to Him, "Lord, show us the Father, and it is enough for us." Jesus said to him, "Have I been so long with you, and yet you have not come to know Me, Philip? He who has seen Me has seen the Father; how can you say, 'Show us the Father'? Do you not believe that I am in the Father, and the Father is in Me? The words that I say to you I do not speak on My own initiative, but the Father abiding in Me does His works. Believe Me that I am in the Father and the Father is in Me; otherwise believe because of the works themselves. (John 14:7-11)