Paul may have in mind the bugle of a city watchman, by which he was to alert the citizens, or the bugle calls that gave orders to different regimens during a battle. In either case, an unclear bugle sound would not spur anyone into action (at least not the right kind of action). A bugle that is not understood misses its purpose.
God’s Word should make a practical difference in his people’s lives. But how can it if they do not understand it?
8 Et si la trompette rend un son confus, qui se préparera au combat?