General use of brethren as meaning is considering archaic and not used. Is it in current usage in the Christian world?
I'm not sure this is a question about Christianity as such. The Oxford English Dictionary for “brother” has an entire section devoted to the religious sense of the word (section A.III), with examples from the 20th and 21st centuries, including the plural “brethren”.
In addition I noticed these secular uses of the word:
- “His attempts at ‘contributing’ were not welcomed by many of his brethren officers.” (Horn & M. Wyczynski in Warrior Chiefs; 2001)
- “Scores of exiled Tibetans..set out on foot towards Tibet to support their brethren who are facing oppression under the Chinese regime.” (Hindustan Times; 2008)