What does Genesis 12:13 mean?

In the previous two verses, Abram presented his fear to his wife Sarai as they prepared to enter the land of Egypt. She was a woman of great beauty. What was to keep the Egyptians from killing him, a vulnerable immigrant seeking food, and taking Sarai for their own?

Now he reveals his plan: "Tell them you're my sister." This was a half-truth. Sarai and Abram were both children of Abram's father Terah, though they had different mothers. Apparently, it was not uncommon at the time to marry a half-sister. Later, God would later forbid such marriages for the people of Israel, but He had not yet done so.

Of course, Abram's statement was also a lie—it was spoken specifically to mislead others about his relationship to Sarai. Sarai was fully Abram's wife. He was asking her to pretend not to be his wife to protect himself, obviously putting her at great risk. It's possible that Abram hoped that being viewed as Sarai's brother would give him the opportunity to reject any marriage proposals she might receive, rather than being a target for a jealous suitor. The following verses will reveal that it won't work out that way.

Abram's failing here began with not trusting the Lord to protect him. God had promised the land to Abram's offspring, along with promises of protection and greatness (Genesis 12:1–3). Since Abram didn't have any children yet, God's promise meant that he would not be killed. Abram was not yet willing to believe God in a potentially dangerous situation.