'Covenant' in the Bible

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.

And this I say, that the covenant, that was confirmed before of God in Christ, the law, which was four hundred and thirty years after, cannot disannul, that it should make the promise of none effect.

Now these facts are about to be used [by me] as an allegory [that is, I will illustrate by using them]: for these women can represent two covenants: one [covenant originated] from Mount Sinai [where the Law was given] that bears children [destined] for slavery; she is Hagar.