At the moment there are two ways you can make this promise. A religious person says:
"I swear by .......... (according to religious belief) that the evidence I shall give shall be the truth the whole truth and nothing but the truth."
A person with no religion says:
"I do solemnly, sincerely and truly declare and affirm that the evidence I shall give shall be the truth the whole truth and nothing but the truth."
The Magistrates' Association are now wondering about changing the wording of the oath to:
"I promise very sincerely to tell the truth, the whole truth and nothing but the truth and I understand that if I fail to do so I will be committing an offence for which I will be punished and may be sent to prison."
Now, that a change is worth considering I do not dispute. Adding the bit about the penalty for lying may be a great idea. The leaving out of the option to swear by whichever God one happens to follow may have its advantages.
But why very sincerely? Is it possible to be slightly sincere, or mostly sincere?
In fact, as time is money (especially, let's face it, when lawyers are involved) why not miss out the sincerely, as well?
I promise to tell the truth.
That's elegant and unambiguous, and bunging sincerity in there just might give people the idea that they're fine as long as they think what they're saying is true.
And that's an invitation not to look too closely into what they're talking about.
Word Not To Use Today Unless Necessary: sincere. This word comes from the Latin sincērus, which means whole, pure, uninjured, truthful.
There's a theory that the word started off being sin cerae, without wax, but all the crustiest etymologists curl their lips with scorn at the very idea.