Note that the title says the STRAIT and narrow. The STRAIT is not necessarily STRAIGHT, and in fact usually is not. A more modern translation might be "cramped and narrow". Myself, I now imagine the strait and narrow as a wildly dangerous trail about the width of one's foot (possibly less) barely hanging on to the side of mountains, twisting and often turning back on itself, often with a sharp drop below it, occasionally going through narrow passes. This is the path to Life.

"The question I am asking these days is, does that other mindset necessarily need to be a Christian one?"

I have considered this question myself - and my natural tendency is to say no, not NECESSARILY.

But as a practical measure? Have we found anything better?

Even if we could, what would the cost of finding it be?

How can we bring this mindset to the masses?

I suspect that a small number of people are naturally inclined toward self-control and pro-social behavior. Of this small group many will do much WORSE with an overly rigid system of self-discipline imposed upon them, and allowances should be made for this. But should we primarily gear society towards the needs of the few or of the many? A curious feature of two of the most successful cultures in the world, England and Japan, is that the masses have a strong tendency to conformity but allowances are made for a handful of intelligent eccentrics who are largely unburdened by mere social conventions.

That said, if we do accept a Christian norm this does NOT mean we have to adhere to every crackpot preacher who makes a wild assertion - and there are many such. Nor do we necessarily have to agree on every point of doctrine. Nonetheless, it seems useful to have a common language of ethical behavior that has stood the test of time and which most are familiar with.

These "self-imposed rules" remind me also of the Protestant insistence on a PERSONAL relationship with God - something which most faiths intermediate through a priest. Perhaps that is part of what has made Protestant nations particularly successful.

"People who do not feel a need for self-discipline or self-control, according to some overarching greater goal, will simply revert to primal desires and doing whatever helps themselves personally, or for their immediate concerns."

I should also caution that I have sometimes observed the propensity for self-dealing to be strong even among people who have devoted their lives to God and who I believe hold their beliefs firmly and sincerely, and who have often exhibited generosity to others. That is, there is no panacea for such tendencies. They can be difficult to avoid even for those who are fully committed. And in those cases I am thankful that these people have their faith, as I wonder what they would be like without any constraints at all. (I particularly got this feeling when I attended a lecture by the retired president of a large bank. I got the feeling that his faith was quite sincere, and yet I also had the impression that it didn't do much to constrain self-dealing behavior.)