"Only fear the LORD, and serve him in truth with all your heart: for consider how great things he hath done for you." (1 Samuel 12:24)
The admonition of Samuel to the people of God after they had asked for a king was to have an awesome fear of God, serve Him with all their hearts, and consider all the great things God had done for them. However, the key ingredient in all of this was to "serve Him in truth." Nothing is more important in service than to do it according to the unchangeable truths of the Word of God. Service must always be aligned with the Scriptures. It should never be done in such a way as to violate a known command of God.
Two Old Testament examples of not serving in truth are found in Exodus and Judges.
In Exodus 32, Moses had been on Mount Sinai for forty days and forty nights. Due to this long delay in coming down, the children of Israel became extremely impatient, asking Aaron to "make us gods, which shall go before us" (v. 1). Aaron obliged them and even built an altar before it in order to offer burnt offerings and peace offerings (vv. 5, 6). Aaron put the name of the "Lord" on an idolatrous practice. He was far from serving in truth. God was not pleased.
In Judges 17, a similar incident happened. A man named Micah stole eleven hundred pieces of silver from his mother which he eventually returned (vv. 1, 2). The mother said, "I had wholly dedicated the silver unto the LORD" (v. 3). Sounds good so far, except she added, "to make a graven image and a molten image" (v. 3). She certainly would not be serving the Lord in truth by doing so. The second commandment was to never make any graven images.
Service and truth go hand in hand. If there is violation of God's truth, the Bible, no matter how much or long one serves, it will never be accepted by God (Matthew 7:15-23). NPS