Adam Clarke on Deuteronomy 30:14

But the word is very nigh unto thee - The doctrine of salvation preached by the apostles; in thy mouth, the promises of redemption made by the prophets forming a part of every Jew's creed; in thy heart - the power to believe with the heart unto righteousness, that the tongue may make confession unto salvation. In this way, it is evident, St. Paul understood these passages; see Rom 10:6-8, etc.