on "Christian" cowardice...

Let me go two ways here:

1.) Individual believers often lack courage-- and when so, it's a lack of faith and/or (head and experiential) knowledge. Courage assumes there's something to fear-- and that's fine; that's life. So, what's the response to fear: courage or cowardice? Key reasons for cowardice for Christians:

a.) lack of trust in God's providence (Do I really believe? I generally live/do in line with what I really believe!);

b.) sins of omission tend to be subtle (People see and call out sins of commission much more often-- their own and others.);

c.) lack of knowledge about what to do in any given circumstance (Related: Christians often quote Joshua 1:6,7,9 to "be strong and courageous", but they don't know and aggressively address what's in 1:8!).

2.) Related but far deeper and wider: many self-styled "Christians" aren't believers; they're "belongers". Religion has its sociological components-- a way of belonging-- and those tend to dominate when religion X is in the majority. Or we might say that they're X culturally but not religiously. In our country, this manifests as "cafeteria Christians"-- picking and choosing among Christian doctrines and practices, without meaningful biblically-Christian community-- as a way of life, with (really) themselves as god.

So, we have been a "Christian" nation in this sociological sense (with a peak in the 1950s). But to your point, we have not been a Christian nation in the religious/biblical sense. What we've seen in the last few decades and esp. the last few years, is the falling away of the belongers from churches-- and a significant increase in self-styling as "nones" rather than "Christian". (Nobody is a "none" in a religious sense, but it's easy to be "none" in the sociological sense.) This trend is generally good news-- probably quite good news, since they're less of a fraud and ironically, more likely to accept the Good News.

If you've read this far, you would like (love?) one of my essays on this latter point-- on "Christianity" as "American Civil Religion".