Deuteronomy 17:10 - Matthew Poole's English Annotations on the Holy Bible

Thou shalt, i.e. thou shalt pass sentence; for he speaks to the inferior magistrates, as was before noted, who were to give sentence, and came hither to be advised about it. Thou shalt observe to do. It is very observable that this place doth not speak of all controversies of faith, as if they were to believe every thing which they should teach; but only of some particular matters of practice and strife between man and man, to which it is plainly limited, Deuteronomy 17:8. And they are not here commanded to believe, but only to do, which is thrice repeated.