It's a fake letter. You notice it says what he is doing wrong, according to the authors personal belief about what is wrong - yet doesn't get into what is the "right way" other than through generic terms as opposed to practical application. The dead giveaway is the phrase "secular music".
This letter is in the spirit of the sort of thing Gobbels would have produced. This thread is really going downhill. I am finding myself wanting to read some of Warrens books to find out what he would say to some of these attacks. It is sort of a "defend the underdog" sort of thing.
I think that would be wise. When I was looking into this whole thing, I started by reading Warren's own work. Frankly, I wouldn't expect a lot of clear commentary or refutation, but without that base you can't evaluate the merits of anything (positive or negative) regarding Warren. And there is much worth evaluating, IMO.
I can appreciate how things appear, particularly if you haven't looked into this much, but FWIW, saying Warren appears the underdog because of a thread on FR is like saying Bill Gates is small fry because a Macintosh BBS contained a post trashing Windows.
Cyrano