John Calvin Commentary Romans 14:18

"For he that herein serveth Christ is well-pleasing to God, and approved of men." — Romans 14:18 (ASV)

For he who in these things, etc. This is an argument drawn from the effect: for it is impossible that the kingdom of God should not fully prevail and flourish in anyone who is acceptable to God and approved by men: he who serves Christ in righteousness with a quiet and peaceful conscience makes himself approved by men as well as by God.

Therefore, wherever there is righteousness, peace, and spiritual joy, the kingdom of God is complete in all its parts. It therefore does not consist of material things.

But Paul states that such a person is acceptable to God because he obeys His will; he testifies that this person is approved by men because they cannot help but bear testimony to the excellence they see with their own eyes. This is not to say that the ungodly always favor the children of God. No, on the contrary, without cause, they often heap many reproaches upon them, defame the innocent with fabricated slanders, and, in short, twist well-done actions into vices by interpreting them maliciously.

But Paul speaks here of honest judgment, free from sullenness, hatred, or superstition.