(1) The moral demand is first put in the negative form: "Be not unequally yoked with unbelievers." The peculiar word ἑτεροζυγοῦντες ("unequally yoked") has a cognate form in Lev. xix. 19, in the law which forbids the breeding of hybrid animals. God has established a good physical order in the world, and it is not to be confounded and disfigured by the mixing of species. It is that law (or perhaps another form of it in Deut. xxii. 10, forbidding an Israelite to plough with an ox and an ass under the same yoke) that is applied in an ethical sense in this passage. There is a wholesome moral order in the world also, and it is not to be confused by the association of its different kinds. The common application of this text to the marriage of Christians and non-Christians is legitimate, but too narrow. The text prohibits every kind of union in which the separate character and interest of the Christian lose anything of their distinctiveness and integrity. This is brought out more strongly in the free quotation from Isa. lii. 11 in ver. 17: "Come out from among them, and be separate, saith the Lord, and touch not anything unclean." These words were originally addressed to the priests who, on the redemption of Israel from Babylon, were to carry the sacred temple vessels back to Jerusalem. But we must remember that, though they are Old Testament words, they are quoted by a New Testament writer, who inevitably puts his own meaning into them. "The unclean thing" which no Christian is to touch is not to be taken in a precise Levitical sense; it covers, and I have no doubt was intended by the writer to cover, all that it suggests to any simple Christian mind now. We are to have no compromising connexion with anything in the world which is alien to God. Let us be as loving and conciliatory as we please, but as long as the world is what it is, the Christian life can only maintain itself in it in an attitude of protest. There always will be things and people to whom the Christian has to say No!
But the moral demand of the passage is put in a more positive form in the last verse: "Let us cleanse ourselves from all defilement of flesh and spirit, perfecting holiness in the fear of God." That is the ideal of the Christian life. There is something to be overcome and put away; there is something to be wrought out and completed; there is a spiritual element or atmosphere--the fear of God--in which alone these tasks can be accomplished. The fear of God is an Old Testament name for true religion, and even under the New Testament it holds its place. The Seraphim still veil their faces while they cry "Holy, holy, holy is the Lord of Hosts," and still we must feel that great awe descend upon our hearts if we would be partakers of His holiness. It is this which withers up sin to the root, and enables us to cleanse ourselves from all defilement of flesh and spirit. St. Paul includes himself in his exhortation here: it is one duty, one ideal, which is set before all. The prompt decisive side of it is represented in καθαρίσωμεν ("let us cleanse": observe the aorist); its patient laborious side in ἐπιτελοῦντες ἁγιωσύνην ("carrying holiness to completion)." Almost everybody in a Christian Church makes a beginning with this task: we cleanse ourselves from obvious and superficial defilements; but how few carry the work on into the spirit, how few carry it on ceaselessly towards perfection. As year after year rolls by, as the various experiences of his come to us with their lessons and their discipline from God, as we see the lives of others, here sinking ever deeper and deeper into the corruptions of the world, there rising daily nearer and nearer to the perfect holiness which is their goal, does not this demand assert its power over us? Is it not a great thing, a worthy thing, that we should set ourselves to purge away from our whole nature, outward and inward, whatever cannot abide the holy eye of God; and that we should regard Christian holiness, not as a subject for casual thoughts once a week, but as the task to be taken up anew, with unwearying diligence, every day we live? Let us be in earnest with this, for surely God is in earnest.