What does Genesis 12:18 mean?

Believing Sarai to be Abram's sister and not his wife, Pharaoh took her as his own wife. In response, the Lord afflicted Pharaoh and his household with plagues, leading to the revelation that Sarai was indeed already married.

Pharaoh is understandably upset with Abram. Not only did Abram withhold the truth that Sarai was his wife, he had received from Pharaoh great gifts of livestock, work animals, and human servants—all while remaining silent about the truth. Pharaoh summons Abram into his presence and begins to question him harshly. The questions continue in the following verse. Abram, apparently, remains silent once again.

The irony of the situation is enormous. Abram's half-truth-and-whole-lie actually caused the very outcome he was trying to avoid. Apparently, even the pagan Pharaoh had enough respect for marriage that he recognized taking a married woman to be a moral crime. Had he known Sarai was Abram's spouse, or so Pharaoh says in verses 18 and 19, he would not have taken her.

We must notice here that Abram does not act honorably in any way in this episode. The Lord, though, remains faithful to His promise. God intends to accomplish His plan through Abram and Sarai no matter what trouble their foolishness leads them into. We should, however, take note that later in life, Abram—then called Abraham—will demonstrate exactly the kind of brave, obedient faith which God expects (Genesis 22; Hebrews 11:17–19).