Unless you specifically mean electrically amplified guitars, you could argue that guitars, and their close relatives lutes and theorbos were definitely used during liturgical services during and prior to the 18th century. Instruments just sustain different pitches, the sounds/timbres they make can't be intrinsically sacred or profane. There are stops on the organ that sound pretty crazy. Listen to the voix humaine on a French classical organ (i.e., French baroque organ music) - it sounds like a herd of goats. There are sounds on the organ that imitate bird calls, etc. The association of guitars with rock music, etc. is decidedly modern.