Genesis 20:3

God (Elohim) came... — From the use of this title of the Deity it has been said that this narrative is an Elohistic form of the Jehovistic narrative in·Genesis 12:10. But we have seen that even in the History of the Fall, where the writer in so remarkable a manner styles the Deity Jehovah-Elohim, he nevertheless restricts Eve and the serpent in their conversation to the name Elohim. With the same care in the application of the names, it is necessarily Elohim who appears to a heathen king; and had the title Jehovah been used it would have been a violation of the narrator’s rule. Moreover, the sole reason for calling that narrative Jehovistic is that in Genesis 12:17 it is Jehovah who plagues Pharaoh for Sarah’s sake. But equally here, Genesis 20:18, it is Jehovah who protects Sarah from Abimelech; in both cases it being the covenant- God, who saves his people from injury.

Thou art but a dead man. — Heb., thou diest, or art dying. Abimelech was already suffering from the malady spoken of in Genesis 20:17, when Elohim appeared to him and warned him that death would be the result of perseverance in retaining Sarah. It was this malady which was the cause of the abstention spoken of in Genesis 20:4; Genesis 20:6.