Genesis 24:4

but thou shalt go unto my country and to my kindred, and take a wife unto my son Isaac. Abraham demanded a solemn oath from Eliezer, in order that this most important mission would be carried out, even if he himself should die soon. By Jehovah, the God of heaven and earth, he had his servant swear, for this was not an ordinary marriage which was contemplated, but a matter of the greatest importance for the kingdom of God, since Isaac was the heir of the divine promise. For that reason also a heathen woman from among the daughters of the Canaanites would not have been acceptable, just as today the marriage of a Christian with an enemy of Christ is always ill-advised, to say the least.