It really MIGHT depend on what the meaning of the word "is" is.

If you think every word has a definition, and definitions are how we know what words mean, consider the word "is" -- which is a pivotal part of every definition -- indeed of the very idea of definition. A bear is a large furry mammal that sometimes walks on it hind legs, etc., etc.

When Bill Clinton said:

"It depends on what the meaning of the word 'is' is. If the--if he--if 'is' means is and never has been, that is not--that is one thing. If it means there is none, that was a completely true statement....Now, if someone had asked me on that day, are you having any kind of sexual relations with Ms. Lewinsky, that is, asked me a question in the present tense, I would have said no. And it would have been completely true."

it was just an example of a legalistic way of dealing with a question -- i.e. it's fine to ignore the intention of a question, even when it is perfectly obvious that you are doing so -- you can split hairs and answer the literal meaning of a poorly or not too carefully worded question, and not say "Oh, but I suppose you meant ...". That's the rule of the game on the witness stand.

But there is one huge and important variation in the way the word "is" is used. If someone says, "I am your friend", or "I am a Christian", or "I am a Muslim", the statement is or should be a promise. Actually it is almost certain to be a mixture of promise and assertion. By the "assertion", I mean you might run through some checklist like "I go to church", "I pray", etc., or "We get together socially every week or so", "I came to help you when you moved".  But the statement {I am your friend/a Christian/a Muslim} would be heard by almost anyone* as having an element of promise, promising for one thing that you will still be  ____ ten minutes from now, tomorrow, maybe forever [your friend/a Christian...]; you are not just making an observation about this moment -- indeed you are not "just making an observation" at all. Unless that element of promise or intentionality  was part of your being as you made the statement [I am your friend...]  -- i.e. if you had a mental reservation like "but I won't be your friend tomorrow", most people with normal brains would consider that untruthful.

The same sort of distinction applies, even more sharply, to any sentence purely about the future (so we're likely to pass from "am/is" to "will" or "shall"). A sentence about the future is either a promise or a prediction, and people are apt to interpret a sentence about oneself in the future as a promise.

Well, maybe, because what you would wish to be a promise is more typically an unholy mixture. A real promise is like when Scarlett O'Hara says "Never Again", and is never quite the same again. A real promise changes the person who makes it. The truth is, nearly everyone is a bit confused about whether they're really making a promise or not. E.g., when you said "I'm going to lose 10 pounds in the next month", it was part promise and part prediction, and as soon as you saw, with a sinking feeling, that the prediction was proving false, whatever promise aspect was in it when you said "I'm going to lose 10 pounds in the next month", that promise aspect probably started to evaporate.

I could cite John Searle (a philosopher) or others, but these words either get you or they don't.

* Now how the hell do I know this, you might ask, but at least consider whether the statement "rings true" for you or not.