Plug your instrument cable into your bass guitar. Connect the other end to the "input" plug of your direct input (DI) box.
Connect your XLR cable into the "output" plug of the DI box. Run the other end into your mixing board.
Connect the mixing board to the PA speakers and turn on the board.
Adjust volume and tone on the mixing board until bass is at an appropriate level. Because of the architectural design, churches tend to have a lot of natural echo and reverb, bass tones can become muddied and indistinct fairly easily. If you find this happening, turn the treble up more than normal so it cuts through the natural echo.