Verse 20 begins Jacob's conclusion. In this case, Jacob has two days of sermon worth to conclude, therefore he must make reference to the major themes for each day's address. He doesn't need to emphasize again the specifics of the promise of the liberty of this land to the righteous as he has just completed it. He does, however, need to refresh the people on the religious covenants that they are renewing. Thus in verse 20 he takes the promises just reviewed and combines them with the specifics of the promises for the redemption from sin. These are presented as the concluding synthesis.
Jacob reminds his congregation to not only remember God, but his promises. He reminds them to lay aside their sins, because the liberty of this people will ultimately rest on their righteousness. He reiterates to them that they are the isles of the sea, and therefore inheritors of the prophecy of Isaiah.