press alt-r / file / new / and paste these two lines into the new remote file and save it
on !*:join:#prayers:{ notice $nick please ask if you need someone to pray with you }
on !*:join:#christians: { notice $nick thank you for visiting us }
each line is a sepearte on join event, with a channel specific message, based apon the channel specific join aka ON JOIN :channel, !* prevents it triggering on u, but allows it to trigger on everyone else.
U may also need to type /!REMOTE ON in the off chance u have turned all remote events off.