You most likely need a bigger PA system. How much you need will depend on the size of the church is you're playing in. You have to take into account the size of the room, the distance of listeners from the stage and so forth.
Larger rooms require more power to produce the same sound pressure levels (which means louder). Keep in mind that power is not everything either, but speaker sensitivity is probably as or more important. You want higher numbers here. I would avoid any speakers that have a sensitivity rating of lower than 96dB. Here's an example of why...
An amplifier pumping 512 watts though speakers that have a 93dB speaker sensitivity rating of 1 watt/1 meter is putting out an actual SPL (sound pressure level) of 120dB. An amplifier putting out 256 watts through speakers with a sensitivity rating of 96dB also puts out 120db actual SPL.
Keep in mind that increasing output by 3dB creates a perceived doubling in volume. That means that if I take that same 256 watt amp and pair it with speakers having a sensitivity rating of 100 db at 1watt/1 meter, I can get 124 dB out of them which is more than enough to create a perceived doubling in volume levels...and they will sound twice as loud as that 512 watt setup! (123dB would be the point of perceived doubling, i.e., 96+3 = 99dB sensitivity rating for speakers to get to 124 dB SPL at 256 watts...so you could do it with 99dB rated speakers.)
What all that means is that good speakers can produce more than twice the loudness of crappy ones depending on the situation and specs you have to play with. If your speakers have a low sensitivity rating, of 91dB, buying speakers with 98dB rating will make your perceived volume double not once, but twice!
Now, 400 watts isn't much, but is it 400 watts at 8ohms or 4ohms? If it is at 8ohms and your amp is capable of putting out more wattage at 4ohms, then you can get more volume by using speakers rated for 4ohm use. In other words, is you have been using speakers rated for 8ohms and 400 watts with a sensitivity rating of 91....and your amp will put out higher wattage at 4ohms....and you switch to 4ohm speaker cabs with a sensitivity of 101dB, you may be shocked to discover your volume levels have doubled three times.
Now, back to the likely scenario. It's a Kustom PA, and so likely you have at your disposal 200 watts per side at 4ohms already. In a situation like that, here's what you should do...
Consider your gig an unmic'd one for all intents and purposes. In other words, balance your levels unmic'd. Your drum kit is the base line...it can't go any louder. BUT...vocals must be loud enough to be heard over them. With a small PA as you have, your goal is to get balanced presentation of the entire band, not extreme volume. Use this and then estimate how much more volume you need. One way to do this is to invest in an SPL meter and then take a reading of your band at various distances from the stage. This will give you numbers to do the math. Just remember that a 3db increase represents a perceived doubling of volume.
You aren't at a rock concert, but are in church, and even though Christian rock bands are supposed to be louder than say a jazz quartet...you don't want concert volume levels making ears bleed 3 rows back.
I would set the drum kit up, center stage if possible of course. Next, have the drummer and vocalist start playing a song while micing the vocalist (forget about micing the drums for now) and pumping his volume up until he is heard over the drums at their loudest (do this to one of your rockingest songs). If the vocalist can't be heard over the drum kit, forget everything else, you need a bigger PA.
If you have no problem getting the vocalist heard above the drums, then start the same song again, and have the bass player join in. Raise his volume to get a good complimentary level with the drum kit. Raise vocal levels a bit to keep things balanced. Repeat by adding guitar one and then two. They should not be individually as loud as the bass. Keep adding vocals to keep them on top.
If you can go through this entire process with only vocals micd, and your band is not loud enough...but you aren't maxing out your PA, you can now do the following.
Put a mic on the kick drum. EQ it and increase its volume a bit to add some presence and 'kick' to the mix. This is probably all you're going to get out of your small Kustom PA. And it of course depends on what sort of amps all the band members have.
If the bass player and guitarists all have 30 watt solid state amps, it's going to be HARD just to be heard over the drums. 30 watt tube amps are a different matter and should be fine. If however you are using direct boxes or PODxt Live or some sort of floor processors straight to the mixer, you will need a larger PA system, that is all there is to it.
The only way to know with certainty what size PA rig you need is to get that SPL meter, stand in different parts of your church and take readings. This means renting a PA rig for the day to use for testing purposes. If for some reason you think you want to feel the bass guitar in your bones on the back row, then your bassist fires up his rig and you mic it and run it through the rented PA and crank the volume until it makes your bones rattle. Look at the sound meter and see what the reading is.
If it is a merciful 90dB and you're 150' from the stage in a room of 'X' sq. feet., take those numbers to a good sound technician. They should get you in the right ballpark.
Most rock bands have way too much volume on stage because they don't understand how to make best use of their equipment. This is probably the reason you get so much feedback. Your PA is underpowered so you are overcompensating with loud stage volumes...and then adding monitors and creating all kinds of interference, feedback.
You only need loud volumes if your PA is wimpy. If you have a strong PA system, you want it to create the volume between you and the audience and have the lowest volumes coming out of your amps on stage you can comfortably get away with. This leaves you room to have your monitored mix being pumped back on stage and being heard clearly without mucking up the stage with feedback and ear-bleed volumes.
There are a lot of conflicting bits of advice out there about how much amp you need for this or that. The bottom line is that it will vary from venue to venue. If you will ONLY play in your church, you are fortunate because you can build your system specifically for that. Most of the time, bands don't have that luxury.
You can basically break your direction towards good sound into two paths: (1) we will have a powerful PA and run it properly or (2) we're scraping by with what we can---which is probably more musicians situations than we'd all care to admit, lol.
If option 1 is at your disposal, fantastic! You can get by with direct boxes and 10 watt amps and mic them all to necessary volume levels.
If option 2 is at your disposal, treat your gig as an unmic'd gig! Try to balance levels among all instruments and get vocals out front, and then raise the overall level as money and equipment allows you by adding more power amps and speakers. Buy the best speakers you can! They do more for your sound than adding another 200 watts.
Perhaps if you post more information, such as the exact type and number of instruments, the size of the room you play in, the maximum and usual numbers of people in that room and such things as "we have a bass player but also a keyboardist that doubles some bass lines with synth parts" tidbits that would point out issues that might need special attention (i.e., you need a lot more power to reproduce bass satisfactorily), perhaps you will get more suggestions as to your particular situation. Mention the size (wattage ratings) of amps your band members are using, etc. as that will help.
Hope this helps some!
