Why intermarriage with Canaanites was forbidden

It says in Exodus:

Observe what I command you today. See, I will drive out before you the Amorites, the Canaanites, the Hittites, the Perizzites, the Hivites, and the Jebusites. Take care not to make a covenant with the inhabitants of the land to which you are going, or it will become a snare among you. You shall tear down their altars, break their pillars, and cut down their sacred poles (for you shall worship no other god, because the Lord, whose name is Jealous, is a jealous God). You shall not make a covenant with the inhabitants of the land, for when they prostitute themselves to their gods and sacrifice to their gods, someone among them will invite you, and you will eat of the sacrifice. And you will take wives from among their daughters for your sons, and their daughters who prostitute themselves to their gods will make your sons also prostitute themselves to their gods. (Exodus 34:11–16, emphasis added)

And again in Deuteronomy:

When the Lord your God brings you into the land that you are about to enter and occupy, and he clears away many nations before you—the Hittites, the Girgashites, the Amorites, the Canaanites, the Perizzites, the Hivites, and the Jebusites, seven nations mightier and more numerous than you—and when the Lord your God gives them over to you and you defeat them, then you must utterly destroy them. Make no covenant with them and show them no mercy. Do not intermarry with them, giving your daughters to their sons or taking their daughters for your sons, for that would turn away your children from following me, to serve other gods. Then the anger of the Lord would be kindled against you, and he would destroy you quickly. But this is how you must deal with them: break down their altars, smash their pillars, hew down their sacred poles, and burn their idols with fire. For you are a people holy to the Lord your God; the Lord your God has chosen you out of all the peoples on earth to be his people, his treasured possession. (Deuteronomy 7:1–6, emphasis added)

All of the seven nations listed except the Perizzites (whose origins are unknown) are descended from Canaan, son of Ham (see Genesis 10:15–19), who was cursed by Noah in the incident recorded in Genesis 9:20–27. This incident and curse on Canaan is the origin story for a lasting conflict between the Hebrew people, who were descendants of Shem, and the Canaanites, who occupied much of the Holy Land at the time Abraham, and later the Israelites, first entered it.

Aside from the implicit directive to avoid intermingling with the descendants of Canaan because they were seen as cursed, a more explicit reason for the prohibition on marrying them given in the above two quotations is that intermarriage with them would cause the Israelites to begin honoring and worshiping their gods, thus forsaking the God of Israel, who demanded their sole worship and obedience.

In a nutshell, then, the Hebrews, and later the Israelites, were forbidden to intermarry with the descendants of Canaan because:

1. Canaan and his descendants were considered cursed.
2. Intermarrying with them would cause them to become unfaithful to their God.

Abraham also wanted to avoid intermarrying with descendants of Canaan

Although the explicit commandment not to intermarry with the descendants of Canaan is recorded in the Bible narrative long after the time of Abraham, many of the commandments given to the Israelites after the exodus from Egypt were codifications of rules that had already long been in place. For example, the law of levirate marriage recorded in Deuteronomy 25:5–10 was already being observed much earlier, during the time of Judah (see Genesis 38:1–11).

We can therefore presume that even during the time of Abraham, intermarriage with descendants of Canaan was not acceptable for Hebrews such as Abraham. This is borne out in the opening lines of the story of Abraham sending his trusted servant to get a wife for his son Isaac:

Now Abraham was old, well advanced in years; and the Lord had blessed Abraham in all things. Abraham said to his servant, the oldest of his house, who had charge of all that he had, "Put your hand under my thigh and I will make you swear by the Lord, the God of heaven and earth, that you will not get a wife for my son from the daughters of the Canaanites, among whom I live, but will go to my country and to my kindred and get a wife for my son Isaac." (Genesis 24:1–4, emphasis added)

Here Abraham specifically mentions the Canaanites among whom he lives as not being suitable to provide a wife for his son Isaac. And though there were other, non-Canaanite nations in the region as well, they, too, would worship other gods, making it too risky to marry his son Isaac to one of their daughters. Instead, he chose to seek a wife for his son from among his own relatives.

The covenant with Abraham

Earlier in the Bible narrative, just before and just after the Great Flood, God had made a covenant with Noah, which would cover him, his descendants, and all living beings on earth. In this covenant, God promised never again to cause a flood that would destroy all living beings. See Genesis 6:18; 9:1-17.

Later, however, God made a more specific covenant with Abram, whom he renamed Abraham. This covenant is first recorded in Genesis 15, and repeated with further detail in Genesis 17. In this covenant, God promises to give to Abraham and to his descendants the land of Canaan. In return, Abraham, all of the males in his household, and all of his descendants and the males in their households must be circumcised as a sign of their covenant with God.

This more specific covenant did not apply to all of humankind, but only to Abraham and his descendants. Further, it did not even apply to all of Abraham's descendants, but only to his designated heir, Isaac, and Isaac's descendants:

And Abraham said to God, "O that Ishmael might live in your sight!" God said, "No, but your wife Sarah shall bear you a son, and you shall name him Isaac. I will establish my covenant with him as an everlasting covenant for his offspring after him. As for Ishmael, I have heard you; I will bless him and make him fruitful and exceedingly numerous; he shall be the father of twelve princes, and I will make him a great nation. But my covenant I will establish with Isaac, whom Sarah shall bear to you at this season next year." (Genesis 17:18–21, emphasis added)

This covenant, then, was very specific to Abraham and his designated heir Isaac, miraculously borne by Abraham's wife Sarah in her old age. Therefore a suitable wife for Isaac must not come from any of these other nations—even the non-Canaanite ones—but must come from as close to Abraham's own family line as possible.

Abraham therefore sent his trusted servant to his immediate relatives living north of the land of Canaan in Aram-naharaim, in the city where his brother Nahor had settled (see Genesis 11:27–32; 24:10). From there, as told in Genesis 24, Abraham's servant got as a wife for Isaac Rebekah, granddaughter of Abraham's brother Nahor, making her Isaac's first cousin once removed.