John Gill Commentary Deuteronomy 15:5

"if only thou diligently hearken unto the voice of Jehovah thy God, to observe to do all this commandment which I command thee this day." — Deuteronomy 15:5 (ASV)

Ver. 5 Only if you carefully hearken to the voice of the Lord your God ,
&c.] In his word, and by his prophets; this being the case, there would be no more poor among them, or however they would be so blessed of God, that they would be capable of releasing the debts of the poor, without hurting themselves and their families:

to observe to do all these commandments which I command you this day ;
a phrase often used to put them in mind of the commands of God, and the necessity of keeping them, their temporal happiness depending thereon.