“This is a painful subject, and so far we have looked only at the injunction. We have not yet considered the reason which our Lord adds to the injunction.
We have just taken the two words, and I trust we shall always remember them. ‘Judge not’. (Matt. 7:1)
As we do so let us thank God that we have a gospel which tells us that ‘while we were yet sinners, Christ died for us’, (Rom. 5:8) that not one of us stands in his own righteousness, but in the righteousness of Christ.
Without Him we are damned, utterly lost. We have condemned ourselves by judging others.
But then God the Lord is our Judge, and He has provided a way whereby we pass ‘from judgment unto life’. (John 5:24)
The exhortation is that we should live our lives in this world as people who have passed through the judgment ‘in Christ’, and who now live for Him and live like Him, realizing that we have been saved by His wondrous grace and mercy.”
–D. Martyn Lloyd-Jones, Studies in the Sermon on the Mount, Second edition. (England: Inter-Varsity Press, 1976), 485–486.