How do we know we should not be judgmental?

Jesus commands us in Matthew 7:1 not to be harsh and condemning towards our brethren. This warning is rooted in the understanding that we are all sinners in need of mercy. As we reflect on our own shortcomings, we become aware of our unworthiness and are reminded to extend the same grace to others that we have received from God. Furthermore, the principle of reciprocity in judgment—'with what judgment ye judge, ye shall be judged'—cautions us that the standard we apply to others will be applied to us by God (Matthew 7:2).