Now therefore fear the Lord, and serve him in sincerity and in truth: and put away the gods which your fathers served on the other side of the flood, and in Egypt; and serve ye the Lord.
And if it seem evil unto you to serve the Lord, choose you this day whom ye will serve; whether the gods which your fathers served that were on the other side of the flood, or the gods of the Amorites, in whose land ye dwell: but as for me and my house, we will serve the Lord.
Acts 2:36-38 (KJV)
Therefore let all the house of Israel know assuredly, that God hath made the same Jesus, whom ye have crucified, both Lord and Christ.
Now when they heard this, they were pricked in their heart, and said unto Peter and to the rest of the apostles, Men and brethren, what shall we do?
Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost.
And God said, Behold, I have given you every herb bearing seed, which is upon the face of all the earth, and every tree, in the which is the fruit of a tree yielding seed; to you it shall be for meat.
Joshua 24:14-15
14 ¶ Now therefore fear the LORD, and serve him in sincerity and in truth: and put away the gods which your fathers served on the other side of the flood, and in Egypt; and serve ye the LORD. 15 And if it seem evil unto you to serve the LORD, choose you this day whom ye will serve; whether the gods which your fathers served that were on the other side of the flood, or the gods of the Amorites, in whose land ye dwell: but as for me and my house, we will serve the LORD.
In this wonderful Bible passage, we find two important words linked together, "sincerity and truth" there's one other Bible passage, in God's preserved word where these two words are linked:
1 Corinthians 5:8
Therefore let us keep the feast, not with old leaven, neither with the leaven of malice and wickedness; but with the unleavened bread of sincerity and truth.
Keeping the feast God's way, is serving God in His righteousness, and in His holiness. Leaven is anything that changes that which is whole. It corrupts, it depraves whatever it is mixed with. In the above verse, Paul writes to the Corinthian believers, of two different leavens, the "old leaven," and "the leaven of malice and wickedness." The old leaven is that leaven, wherein people attempt to worship the Lord with a divided heart. A "whorish heart" seeks not to glorify God, but its own egocentric desires, a prostitute does what she does for self-centered reasons, a whore does it for the profit.
Ezekiel 6:9
And they that escape of you shall remember me among the nations whither they shall be carried captives, because I am broken with their whorish heart, which hath departed from me, and with their eyes, which go a whoring after their idols: and they shall lothe themselves for the evils which they have committed in all their abominations.
Today, those with the "whorish heart" are people who seek after God any other way, other than Biblical Christianity. People whore themselves out to false religions for a variety of selfish reasons. Maybe it's because of what friends or family might think of them if they walked away from it, they may remain in the false religion, because of fear of retaliation from the 'spiritual leader.' or they may simply be making monetary profit selling books, CD's or online courses.
Whatever the reason may be, trying to serve God any other way other than Bible Christianity is serving God a false way, and God wants His people to hate every false way.
Psalms 119:104
Through thy precepts I get understanding: therefore I hate every false way.
Psalms 119:128
Therefore I esteem all thy precepts concerning all things to be right; and I hate every false way.
The other leaven, " the leaven of malice and wickedness" are the false prophets and false teachers, who have come into the Church 'unawares' and intentionally lead people to hell with their heresies.
2 Peter 2:1
But there were false prophets also among the people, even as there shall be false teachers among you, who privily shall bring in damnable heresies, even denying the Lord that bought them, and bring upon themselves swift destruction.
Whatever leaven has crept into the house of God, all Bible believing Christians must (with God's help and with His preserved word) expose, shun and purge it out of their midst. In that way, their fellowship can remain serving the Lord in 'sincerity and in truth.'
May the Lord bless you and yours.