Christians should have souls that are in harmony. A common disposition will follow from unity of thought and affection. This is a symphony of the soul. If a note is struck, the same note will answer when in key with another instrument. Everything has its note.
Musical instruments are sensitive and readily get out of tune.
A common tuning instrument is necessary to keep instruments in harmony. Christians should have a harmony of souls with each other. Our common tuning instrument with each other is God himself. If we each individually keep our lives in tune with God, we can be in tune with each other. The discord will become lost in the orchestra of glorious praise to God.