1. Either (a) the purposes God sets for our lives are significant because God wills them, or (b) God wills them because they’re significant.
2. If (a), then what counts as a significant life is arbitrary.
3. If (b), then what counts as a significant life is independent of God
4. Therefore, what counts as a significant life is either arbitrary or independent of God.
I noticed this argument and found it worth sharing: