'Sincerity' in the Bible

Now therefore fear Jehovah, and serve him in sincerity and in truth; and put away the gods which your fathers served beyond the River, and in Egypt; and serve ye Jehovah.

wherefore 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.

For our glorifying 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 to you-ward.

For we are not as the many, corrupting the word of God: but as of sincerity, but as of God, in the sight of God, speak we in Christ.

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