If you are in a band, be it in church or not, powered monitors are very important aside from having good quality musical instruments. If you are a vocalist, it is a must that you can hear yourself when you sing—for musicians, at least you can hear the volume of your instrument while playing. Monitors could be costly, but if you are aiming for your band to sound good, it is definitely worth the investment.
Without good monitors, instrumentalists could not hear the other instruments being played, or they could not hear their own so they would not sound good at all. For musicians/ bands, it is important to note that good sound mixing and the investment of high quality equipment coupled with talent is guaranteed to make you sound great!