Do you promise to tell the truth, the whole truth, and nothing but the truth, so help you God?
Here's something amusing I just found out: apparently making a promise in God's name - such as in the stereotypical US courtroom line in the title of this entry - is (ironically) un-Christian, because as Jesus said:
Again, you have heard that it was said to the people long ago, "Do not break your oath, but keep the oaths you have made to the Lord." But I tell you, Do not swear at all: either by heaven, for it is God's throne; or by the earth, for it is his footstool; or by Jerusalem, for it is the city of the Great King. And do not swear by your head, for you cannot make even one hair white or black. Simply let your 'Yes' be 'Yes,' and your 'No,' 'No'; anything beyond this comes from the evil one.