In this text, the Messiah, and by extension the Messiah's people, are concerned that their efforts will be in vain or useless. Paul's emphasis is for the Philippians to see what is at stake in their joy, unity, and lack of grumbling and disputing.

Do all things without murmuring and disputing?

Do all things without murmurings and disputings: That ye may be blameless and harmless, the sons of… | by Keith McGivern | Medium.

Why is grumbling a sin?

Grumbling is a sin

As the mighty Judge, he will return at any moment and hold all the world to account for what they have said and done in this life.