But (de) - Godet "The but contrasts the incompetent judgment of a brother, with the judgment of this one Lord." (Romans 14:1-15:13 Directions Regarding a Difference of View)
We need to recall what the Lord Jesus said to the Pharisees who wanted to stone an adulterous woman...
Let the world condemn him if it will, but simply because we are intimately related in Christ let us get alongside our brother and love him or at least overlook his failures without seeking to condemn him for his faults. Let no man judge another because one day, in the light of the judgment we have passed on others, every one of us must stand before the judgment seat of Christ (Mt 7:2). Silence (forbearance) the censorious tongue is good advice for those who desire to walk worthy of their calling (Ep 4:1-3). Cast the burdens of the misjudged motive and the misunderstood action at the feet of the Lord and leave them there until that day when He shall vindicate, for vengeance is His and He will repay (Ro 12:19, He 10:30, Dt 32:35, 43, Ps 94:1, 2, Nah 1:2-3, Lev 19:18, cp Pr 24:17-18).
The LORD...is coming, or He is coming to judge the earth. He will judge the world in righteousness, and the peoples in His faithfulness. (Psalm 96:13) (Spurgeon's note)