'Sincerity' in the Bible

"Now therefore fear Yahweh, and serve him in sincerity and in truth. Put away the gods which your fathers served beyond the River, in Egypt; and serve Yahweh.

Therefore let us keep the feast, not with old yeast, neither with the yeast of malice and wickedness, but with the unleavened bread of sincerity and truth.

For our boasting is this: the testimony of our conscience, that in holiness and sincerity of God, not in fleshly wisdom but in the grace of God we behaved ourselves in the world, and more abundantly toward you.

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

I speak not by way of commandment, but as proving through the earnestness of others the sincerity also of your love.