Contendah, my Friend,
If you walked into a church, any church in any city, to address them, I imagine you would begin be greeting them, "Brothers and sisters. . . " Most folks would.
Do YOU honestly believe that every church is 100% true believers? Are you that naive? Well, neither was Paul. Yet, he wanted to give them a Christian greeting, so he began by addressing them as "Brethren."
Yet, Paul shows that he was not as naive as you, for this is what we read:
12. Take heed, brethren, lest there be in any one of you an evil heart of unbelief in departing from the living God.
13. But exhort one another daily,. While it is called “Today,” lest any of you be hardened through the deceitfulness of sin.
14. For we have become partakers of Christif we hold the beginning of our confidence steadfast to the end....”
Seems that old Paul knew there were some sitting there who were just social seat-warmers.
God bless, have a wonderful, blessed day,
Bill