At the time Paul wrote to the Philippians they were enduring a time of persecution and suffering (see 1:29), yet he instructs them to do everything without grumbling, or as some translations put it, without murmuring.
Murmuring is the opposite of thankfulness. When we murmur, grumble, and complain, we reveal a heart that is not grateful for the goodness of God. Murmurs may come from the mouth, but they are a problem of the heart! God considers murmuring a serious sin. In 1 Corinthians 10, grumbling is put right alongside immorality and idolatry as things that displease God. Why is complaining such a serious offense? Because it completely undercuts our witness to the world. We can’t proclaim to the lost that our God is good and great while we grumble about what he has given us.
On the other hand, as Paul points out to the Philippians, when we offer thanks to God even in the difficult times, we stand out from others like lights shining in the darkness, and everyone knows that we are the children of God. For the sake of the gospel, let’s be sure our hearts are overflowing with thanksgiving, not with murmuring and complaining