[re: Washington's Birthday being called 'President's Day' as an example of "secularized language"]

Washington was a prominent Christian whose habit of praying and appealing to God for assistance is well-known. He was a kind of saint, and the downplaying of his (indisputable) significance is due to his Christianity. If he had been an atheist then you'd see the opposite effect.

"What kind of saint was George Washington?"

The kind that wins wars. There have been other examples. Have you heard of Joan of Arc?

"So which churches DO recognize Washington as a saint or other type of sacred being?"

Perhaps the church of Satan does, or perhaps atheists simply dislike Washington. Wouldn't that be enough to explain the misnaming of the holiday as described in this entry?
...
Atheistic bias, like other forms of bias, seeks to downplay and minimize Christianity. Washington was a leading, accomplished Christian. Lincoln far less so, other presidents even lesser still, and the office of president not Christian at all. An atheistic bias pushes language from the Christian recognition to the more atheistic substitute.