38 A mixed multitude also went up with them, and flocks and herds—a very large number of cattle. 39 They baked cakes of bread without yeast using the dough they had brought from Egypt, for it was made without yeast. Because they were thrust out of Egypt and were not able to delay, they could not prepare food for themselves either. 40 Now the length of time the Israelites lived in Egypt was 430 years. 41 At the end of the 430 years, on the very day, all the regiments of the Lord went out of the land of Egypt. 42 It was a night of vigil for the Lord to bring them out from the land of Egypt, and so on this night all Israel is to keep the vigil to the Lord for generations to come.
15 Brothers and sisters, I offer an example from everyday life: When a covenant has been ratified, even though it is only a human contract, no one can set it aside or add anything to it. 16 Now the promises were spoken to Abraham and to his descendant. Scripture does not say, “and to the descendants,” referring to many, but “and to your descendant,” referring to one, who is Christ. 17 What I am saying is this: The law that came 430 years later does not cancel a covenant previously ratified by God, so as to invalidate the promise. 18 For if the inheritance is based on the law, it is no longer based on the promise, but God graciously gave it to Abraham through the promise. 19 Why then was the law given? It was added because of transgressions, until the arrival of the descendant to whom the promise had been made. It was administered through angels by an intermediary.
Notes and References
"... Paul applied the principle of the permanence of faith by affirming that a covenant made so long before could not possibly be altered by a later giving of the Law. The Law was given 430 years after the promise. When did that lengthy period of time begin? Some have suggested it began with Abraham, in which case the 430 years included the Israelites' time of about 200 years in Canaan and about 200 years in Egypt. The Septuagint supports this view, but this conflicts with the clear statement in Exodus 12:40 that the Egyptian sojourn was 430 years. Another suggestion is that the period began with the confirming of the Abrahamic Covenant with Jacob (Genesis 35:9-12). A third and perhaps best view is that the period began with the final confirmation of the covenant to Jacob (given in Genesis 46:1-4). Accordingly the 430 years went from the end of one era (the Age of Promise) to the beginning of another (the Age of Law). This seems to fit best with Exodus 12:40 ..."
Walvoord, John F. The Bible Knowledge Commentary: An Exposition of the Scriptures (pp. 598-599) Victor Brooks, 1983