To the best of my knowledge (recently went through CCW training in CO), churches are not exempt from the requirement to conspicously post signs banning fire arms on the premise. Only a few types of establishments have implicit "no-carry" rules (government buildings, all schools, bars).
For the record, posting a "No fire arms allowed" rule is little more than an invitation for the armed criminal to come in and do as he/she pleases. This last Sunday, I was in church, within only a few miles of one of these churches that was struck. We were ajourned early and told to go home, as there was concern by the church leadership that those involved might be randomly targeting churches in the area. This bothered me, as it seemed like giving into fear; however, as I was not armed and did not know if anyone else was armed, I can understand the precaution.