So whatever you wish that others would do to you, do also to them, for this is the Law and the Prophets.” (Matthew 7:12)
It is an exhortation to his [Christ’s] disciples to be just, and contains a short and simple definition of what justice means. We are here informed, that the only reason why so many quarrels exist in the world, and why men inflict so many mutual injuries on each other, is, that they knowingly and willingly trample justice under their feet, while every man rigidly demands that it shall be maintained towards himself.
Where our own advantage is concerned, there is not one of us, who cannot explain minutely and ingeniously what ought to be done. And since every man shows himself to be a skilful teacher of justice for his own advantage, how comes it, that the same knowledge does not readily occur to him, when the profit or loss of another is at stake, but because we wish to be wise for ourselves only, and no man cares about his neighbours? What is more, we maliciously and purposely shut our eyes upon the rule of justice, which shines in our hearts. Christ therefore shows, that every man may be a rule of acting properly and justly towards his neighbours, if he do to others what he requires to be done to him. He thus refutes all the vain pretences, which men contrive for hiding or disguising their injustice. Perfect justice would undoubtedly prevail among us, if we were as faithful in learning activecharity, (if we may use the expression,) as we are skilful in teaching passive charity.
Source: John Calvin | Monergism