Joshua commanded Israel to “fear the Lord, and serve him in sincerity and in truth”, Jos 24:14.  Not just in sincerity.  Not just in truth.  But in the combination of sincerity and truth.

Sincerity is honesty of mind; freedom from hypocrisy.  Sincere means:  without deceit, pretense [a false profession]; free of dissimulation; being genuine, the same in actual character as in outward appearance.

The trouble with Israel at this time is that they had been influenced by their fathers to worship strange gods along with the Lord God.  When Joshua gave them this command he told them to put away the strange gods that their fathers had served back in Terah’s day.  And to put away the strange gods their fathers had served in Egypt.  You might remember that when Jacob left Laban, Rachel had stolen Laban’s gods.

There was no way that they could serve the Lord while they were still hanging on to their strange gods.  Look carefully at Jos 24:18-24.  In v.18, they professed that they would ALSO serve the Lord.  Meaning that they were going to try to serve him and keep their gods.  Joshua replied in v.19, “Ye cannot serve the Lord… he is a jealous God”.

The people insisted that they were going to serve the Lord and Joshua insisted that they had to put away their gods.  At the end of this discourse, the people said, “The Lord our God will we serve, and his voice will we obey”.  To serve the Lord in sincerity, they would have to serve him without serving their gods.  And to serve him in truth, they would have to obey him.  Sincerity and truth.  They tried to serve him in truth without serving him in sincerity.  And they couldn’t do it.

The Pharisees attempted this dissimulation.  And it didn’t work.  They ended up changing truth to tradition and sincerity to hypocrisy, Matt 15:1-9.  The Catholics have followed their pattern.

Muslims appear to be so sincere with their zeal and their attendance at prayer five times each day.  But people who live and work among them testify that adultery and wife beating are common among Muslims.  They aren’t serving God in truth or in sincerity.  Jehovah’s Witnesses and Mormons may be very sincere, maybe, but they certainly aren’t serving the Lord in truth.

But what about us?  We must be careful that we are not guilty of serving the Lord in truth without serving him in sincerity.  In other words, we know the truth and believe the truth and don’t worship a prophet, or a society, or a pope, or a goddess, or any such strange god.  We serve the Lord in truth.  Yet, we must also serve him in sincerity.  We must serve him in sincerity and truth.

Notice in Gal 2:11-14 how it’s possible to serve the Lord in truth but not in sincerity.  God chose Peter to preach the gospel to the Gentiles when he sent him to Cornelius and his kinsmen, Acts 15:7. So, if anyone knew the truth about salvation among the Gentiles, it was Peter.  Yet, when Peter and other Jews, including Barnabas, were among the Gentiles in Antioch, they were carried away with dissimulation.  They ate with the Gentiles until Jews from Jerusalem showed up. And then they separated themselves to appear that they had been keeping the law.

Do you see?  You know the truth and believe the truth.  But if you serve the Lord here, and out there you serve something else or someone else, then you aren’t serving the Lord in sincerity and truth.

For example, we are to obey men, but we are to serve the Lord, Eph 6:5-8.  We are to work to eat and to be able to give, Eph 4:28.  But we are not to serve mammon, Matt 6:24.  Covetousness is idolatry, Eph 5:5.

Conclusion: It is very important that we learn from our text that God wants us to not only serve him in truth.  But he also wants us to serve him in sincerity and truth.  Many Christians today are trying to serve him and something else at the same time.  It doesn’t work.