'Sincerity' in the Bible

Therefore, let us observe the feast, not with old yeast or with the yeast of malice and evil but with the unleavened bread of sincerity and truth.

For this is our confidence: The testimony of our conscience is that we have conducted ourselves in the world, and especially toward you, with God-given sincerity and purity, not by fleshly wisdom but by God’s grace.

For we are not like the many who market God’s message for profit. On the contrary, we speak with sincerity in Christ, as from God and before God.

Slaves, obey your human masters with fear and trembling, in the sincerity of your heart, as to Christ.