John Calvin on Matthew 5:28

28. Whoever shall look upon a woman. The design of Christ was to condemn generally the lust of the flesh. He says, that not only those who have seduced their neighbors’ wives, but those who have polluted their eyes by an immodest look, are adulterers before God. This is a synec-doche: “C'est une facon de parler qu'on appelle Synecdoche, quand on prend une artie our le tout.” — “It is a way of speaking which is called Synecdoche, when a part is taken for the whole.” for not only the eyes, but even the concealed flames of the heart, render men guilty of adultery. Accordingly, Paul makes chastity (1 Corinthians 7:34) to consist both in body and in mind. But Christ reckoned it enough to refute the gross mistake which was prevalent: for they thought that it was only necessary to guard against outward adultery. As it is generally by the wantonness of the eyes that temptations are presented to the mind, and as lust enters, as it were, by that door, Christ used this mode of speaking, when he wished to condemn lust: which is evident from the expression, to lust after her. This teaches us also, that not only those who form a deliberate purpose of fornication, but those who admit any polluted thoughts, are reckoned adulterers before God. The hypocrisy of the Papists, therefore, is too gross and stupid, when they affirm that lust is not a sin, until it gain the full consent of the heart. But we need not wonder, that they make sin to be so small a matter: for those who ascribe righteousness to the merit of works must be very dull and stupid in judging of their sins.