Encouraging someone to believe you

Encouraging someone to believe you
That’s the truth.
That’s the honest truth.
That’s the honest-to-goodness truth.
Honest to goodness.
That’s the truth, the whole truth, and nothing but the truth.
Cross my heart and hope to die. (juvenile)
Would I lie?
Would I lie to you?
Why would I lie?
I swear.
I swear to you.
I swear on a stack of Bibles. (mild oath)
I swear on my mother’s grave. (informal)
I swear to God. (mild oath)
May God strike me down if I am not telling you the truth. (mild oath)
 That’s the gospel truth, (informal)