You don't lean on your own understanding. You go directly to the Lord in prayer and put it in His hands. What if, say, they really aren't a believer, but think they are and just need the unjudgemental/unconditional love of the congregation that knows they can't play (and they know they can't play) and through that they finally understand His unconditional love to sinners, who never could and never will play the "righteous" organ well? There are several other scenarios we could come up with that it would God's will not to fire the organist - including the need for the congregation to be more loving toward the rotten playing. The bottom line is that God knows the hearts - theirs and ours and we don't know His will in a situation that will bring Him glory in their hearts. But He does!
For everything there is a season, and a time for every matter under heaven:
a time to be born, and a time to die;
a time to plant, and a time to pluck up what is planted;
a time to kill, and a time to heal;
a time to break down, and a time to build up;
a time to weep, and a time to laugh;
a time to mourn, and a time to dance;
a time to cast away stones, and a time to gather stones together;
a time to embrace, and a time to refrain from embracing;
a time to seek, and a time to lose;
a time to keep, and a time to cast away;
a time to tear, and a time to sew;
a time to keep silence, and a time to speak;
a time to love, and a time to hate;
a time for war, and a time for peace.
What gain has the worker from his toil? I have seen the business that God has given to the children of man to be busy with. He has made everything beautiful in its time.
But not everyone can play the organ. Psalm 33 says to "play skillfully".