They will turn away thy son from following Me. Deuteronomy 7:4
The question of marriage is repeatedly considered in these chapters, and never once is it supposed that the Israelites might bring a heathen partner to the faith of God's elect; but it is always insisted that the heathen husband or wife will subvert the faith of the child of Abraham. "Thou shalt not make marriages with them; for they will turn away thy son from following Me, that they may serve other gods For thou art an holy people unto the Lord thy God."
The same law holds still. You may suppose that by marrying the ungodly and irreligious you will be able to convert them to your way of thinking; but you must remember that regeneration is the work of the Holy Ghost, and He is not likely to lend His aid in regeneration whilst you are acting in defiance of His distinct prohibitions. The command of Christ is so clear and positive against His followers entering into an unequal yoke with unbelievers, that it simply leaves no option for the obedient. With the child of God, marriage must be "only in the Lord."
In order to make these marriages impossible, Israel was bidden to destroy the nations of Canaan. Separation from their society and practices was thus enforced. The slaughter seemed ruthless; but there was no other way of preserving intact the chosen race, as a peculiar people unto the Lord. Our separation also must be strict even to the extreme. If we would keep our young people from worldly alliances, we must begin with their amusements and companionships. There should be every endeavor to promote their happiness and interests; but we must very carefully guard the young plants from the blight of worldliness.