Only fear the LORD, and serve him in truth with all your heart: for consider how great things he hath done for you. Only fear the Lord - Know, respect, and reverence him.
Serve him - Consider him your Lord and Master; consider yourselves his servants.
In truth - Be ever honest, ever sincere; with all your heart - have every affection engaged in the work of obedience; act not merely from a principle of duty, but also from a pious, affectionate sense of obligation. Act towards your God as an affectionate child should act towards a tender and loving parent.
Consider how great things - Review the history of your fathers, review your own life; see what interpositions of power, mercy, goodness, and truth, God has displayed in your behalf! Has he not daily loaded you with his benefits?