1 And Yahweh said unto Moses, Yet will I bring one plague more upon Pharaoh, and upon Egypt; afterwards he will let you go hence: when he shall let you go, he shall surely thrust you out hence altogether.
The tenth and culminating plague was unique. Exemption there certainly was to be for Israel once again, but this time not automatically. The plague was to be the dramatic denouement of the struggle between Pharaoh and Yahweh for possession of Israel.
As the moon of the month Abib came to the full so also would God's threat-
" If thou refuse to let him go, behold, I will slay thy son, even thy first born" .
The plague was to be final and decisive proof that
"the Lord doth put a difference between the Egyptians and Israel".
Israel had therefore to be true to, itself: its exemption from the plague was to be contingent on willing conformity to divine instructions. This was a salutary reminder to the Israelites that, though they were the objects of God's special choice and interest, yet in reality they deserved deliverance from destruction no more than did the Egyptians; like circumcision it was witness that they enjoyed their privileged position not of right, but solely in consequence of God's gracious choice of them, and
"because he would keep the oath which he had sworn'.' (Deut. 7 : 6-8).
Law and Grace Ch 4