'Sincerity' in the Bible

For our proud confidence is this: the testimony of our conscience, that in holiness and godly sincerity, not in fleshly wisdom but in the grace of God, we have conducted ourselves in the world, and especially toward you.

For we are not like many, peddling the word of God, but as from sincerity, but as from God, we speak in Christ in the sight of God.

I am not speaking this as a command, but as proving through the earnestness of others the sincerity of your love also.