John Calvin Commentary Deuteronomy 26:16

"This day Jehovah thy God commandeth thee to do these statutes and ordinances: thou shalt therefore keep and do them with all thy heart, and with all thy soul." — Deuteronomy 26:16 (ASV)

This day the Lord your God. He again reminds them that God is the author of the Law, so that His majesty might impress them more. Not only this, but since the Law was specially delivered to them, its observance was all the more required of them.

Therefore, he earnestly exhorts them to apply their hearts to those things which God had commanded them to keep, because people grow careless in their duties unless they are often stirred up. For undoubtedly, God indirectly rebukes the people’s indifference by so often calling them to obedience.

By the words with all your soul is meant serious understanding and carefulness, as well as sincerity, free from all disguise and deceit. For nothing is more displeasing to God than hypocrisy, because He sees the heart.

If anyone objects that it was futile to demand of them what no mortal can perform—namely, to keep the Law with all their heart—I reply that all the heart is opposed to a double or divided heart. It is equivalent to an entire heart, or one altogether without deceit. Although (as we shall see later) it is not absurd to propose an objective for believers at which they are to aim, even if they may not attain it as long as the weakness of the flesh hinders them.