Why were the Jewish families safe in Egypt while the death angel passed over?
The Jewish families in Egypt were safe and secure while the death angel passed over because the blood was on the door. Their safety rested not in their own efforts but in the blood on the door, which stood between them and the judgment occurring throughout Egypt.
Les explains that the safety of the Jewish families in Egypt rested on the blood on the door. He compares this to Noah: the ark was sealed with pitch, which in the Hebrew is atonement, and that atonement sealed out the flood waters so Noah and his family were totally safe. In the same way, Israel stood at the little kitchen table in their huts in Egypt while the death angel flew over and wailing and weeping carried on in Egypt, yet those Jewish families could stand safe and secure because the blood was on the door. Later, at the Red Sea, God told them to stand still and see the power of God, which was the opening of the Red Sea. Les points out that Paul had to find himself exactly like Israel: there was only one place of safety, the blood of Christ, and the question was how he would appropriate it.
And the blood shall be to you for a token upon the houses where ye are: and when I see the blood, I will pass over you, and the plague shall not be upon you to destroy you, when I smite the land of Egypt.