John Calvin Commentary Exodus 12:23

"For Jehovah will pass through to smite the Egyptians; and when he seeth the blood upon the lintel, and on the two side-posts, Jehovah will pass over the door, and will not suffer the destroyer to come in unto your houses to smite you." — Exodus 12:23 (ASV)

For the Lord will pass through. He forbids them to go out during the night, so that they would not mix with the Egyptians, but commands them to remain quietly under the protection of the blood. By this sign they were warned that they also were exposed to destruction if they did not separate themselves from the unbelievers under the safeguard of the blood.

Afterward, the promise is added that, provided this was done, the angel would pass them over and not harm them, because God would recognize the houses so marked as His own. Therefore, it is again repeated that only those who would not neglect to sprinkle themselves with it would be safe by the blessing of the blood; because faith alone confers on us the salvation that is obtained through the slaughter of the victim.

The angel, whom God had appointed to afflict Egypt, is here undoubtedly called "the destroyer"; and, although He often carries out His judgments through evil angels, it can be gathered from other passages that this was one of the elect angels, who also was the minister of the people's deliverance under Christ as the Head.