Deuteronomy 12:28
Observe and hear all these words which I command thee, that it may go well with thee, and with thy children after thee for ever, when thou doest that which is good and right in the sight of the LORD thy God. Observe and hear all these words which I command thee, that it may go well with thee, and with thy children after thee for ever, when thou doest that which is good and right in the sight of the LORD thy God.
Observe and hear all these words means careful listening joined to full obedience to everything God requires. Doing what is good and right in the sight of the Lord brings lasting well-being to those who obey and to their children after them forever.
The law was given for the people's protection and highest good, so that obedience would make them a pure and wise people and a witness to other nations. In its setting this heeding included refusing to imitate the idolatrous religious practices of the displaced nations and refusing to alter God's instructions by adding to or subtracting from them.
The command allows no liberty to set aside any requirement or to follow what seems right by human judgment, desire, or device, but makes agreement with God's will the proper standard and measure of conduct. Partial obedience does not satisfy it even when neglect is defended by a plausible motive, as shown by Saul's failure to carry out God's command against Amalek. God's words are to be received and passed on as His words, not as human inventions.