'Sincerity' in the Bible

For this is what we boast about: Our conscience testifies that we have conducted ourselves in the world with pure motives and godly sincerity, without earthly wisdom but with God's grace especially toward you.

At least we are not commercializing God's word like so many others. Instead, we speak with sincerity in the Messiah's name, like people who are sent from God and are accountable to God.

Slaves, obey your earthly masters with fear, trembling, and sincerity, as when you obey the Messiah.