Galatians 3:15
Brethren, I speak after the manner of men; Though it be but a man’s covenant, yet if it be confirmed, no man disannulleth, or addeth thereto. Brethren, I speak after the manner of men; Though it be but a man’s covenant, yet if it be confirmed, no man disannulleth, or addeth thereto.
Paul reasons after the manner of men that even a man's covenant, once it is confirmed, cannot be disannulled by anyone nor have anything added to it, and he applies that principle to place 430 years between God's covenant promise to Abraham and the giving of the law at Sinai. That 430-year period began with the promise when Abraham was called to leave Ur, not with Israel's entry into Egypt, leaving about 215 years for the sojourn in Egypt. The 400 years of sojourning and affliction spoken of in Genesis 15:13 is a separate and later period that begins about 30 years afterward, around the time Ishmael persecuted Isaac.
The covenant of grace given to Abraham was confirmed to include the Gentiles, so that Gentiles who knew nothing of circumcision are brought under it. The blessings once given to the Jewish nation are given equally to Gentiles who accept Christ as Saviour. The teaching of Galatians 3:15-25 on the covenants is plain light and clear, convincing evidence that every candid, unprejudiced mind would receive. Attempts to produce a different position waste investigative power and repeat hostile scrutiny that watched words in order to twist them into desired meanings, making false issues to cloud and mislead minds. Turning from its plain light comes from fear of having to accept what it teaches about the law, and no one is prepared to rightly establish or refute a position on it until truly converted before God.