Therefore fear the Lord, and serve him in truth and with your whole heart, for you have seen the great works which he hath done among you.

only, fear ye Jehovah, and ye have served Him in truth with all your heart, for see that which He hath made great with you;

Only go in the fear of the Lord, and be his true servants with all your heart, keeping in mind what great things he has 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. Acts 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?

Only fear the LORD, and serve him in truth with all your (o) heart: for consider how great [things] he hath done for you.
(o) Unfeignedly, and without hypocrisy.

Only fear the Lord, and serve him in truth with all your heart,.... Fear him not with a servile fear, which is before dehorted from, but with a filial fear, a reverential affection for God; and includes the whole of religious worship, internal and external; explained further by serving him according to the truth of his word, and in a cordial, sincere, and affectionate manner; and if this was wanting in them, he suggests that his prayers and instructions would be of little avail, and not to be depended on:
for consider how great things he hath done for you; in bringing them out of Egypt: settling them in the land of Canaan; giving them his laws, statutes, commands, and ordinances; sending prophets unto them, and raising up judges for them, and bestowing all good things on them, in nature, providence, and grace; though some restrain this to the great thing he had done that day, to convince them of their sin, and by which they were returned to the Lord, namely, the violent storm of thunder; which wonderful instance of the power of God, and token of his displeasure against them, they were to lay up in their minds, and not forget, that it might be a means of preserving them from sin for the future.

Lastly, he repeats once more his admonition, that they would continue stedfast in the fear of God, threatening at the same time the destruction of both king and people if they should do wrong (on 1-Samuel 12:24, see 1-Samuel 7:3 and Joshua 24:14, where the form יראוּ is also found). "For see what great things He has done for you" (shown to you), not by causing it to thunder and rain at Samuel's prayer, but by giving them a king. עם הגדּיל, as in Genesis 19:19.

Only, &c. - Otherwise neither my prayer nor counsels will stand you in any stead.