Paul goes on at great length about the danger of criticism; the danger of thinking you have all the answers, spiritually, and other people need to just "get with the program." He warns against flaunting your freedom or looking down on others because they do things differently. He suggests withholding your rights for the benefit of the believer next to you who might be confused by your actions. "If you serve Christ with this attitude, you will please God." (v18)
Two- and Three-Part Inventions are tricky to play. (Truth-be-told, I could never play a Three-Part Invention well!) These types of pieces were composed more as an exercise than for their melodic sound, because it requires the pianist to give opportunity to 3 separate voices, working together, flowing in and through and around each other. A Two Part Invention works a little easier, because the two parts come from your two hands, though it takes practice to have them work so independently of each other. For 3 parts, each hand has their own part and then on top of that, they share the work of the third part. The musician who can play this well (that would not be me), can give a strong voice to each part when it is their turn to play the lead while the other parts take a back seat, but are still supporting.
"So then let us aim for harmony in the church and try to build each other up." (v19)
The disciplined disciple is sensitive to the needs and feelings of others and desires to see them flourish in their walk with the Lord. It means not pushing our own ideas, insisting that our way is the only way. It means extending grace to others and being willing to learn from others (for perhaps, they are also extending grace to you!). The individual's relationship with the Lord is very personal - which is why we hold such deep convictions about the things the Lord has revealed to us; which is why it is hard to understand where someone else is coming from.
When I was a teen figuring out my faith, I honestly believed that if every Christian (true Christ follower) just really prayed and sought God's will, we would all come to the same conclusion and the Church would be universally unified. Scripture is clear that this will not be the case in the Kingdom of now-and-not-yet. "Who are you to condemn God's servants? They are responsible to the Lord, so let him tell them whether they are right or wrong. The Lord's power will help them do as they should." (v4)
Part of being in the Church on this earth requires the skill and practice that it takes to play Three-Part Invention: knowing when to step forward and when to make room for others to step forward, and in doing so, we can "join together with one voice, giving praise and glory to God." (15:6)
"So accept each other just as Christ has accepted you;
then God will be glorified."