What does it mean to be merciful to a sinner?  The first three of the seven spiritual works of mercy are to counsel the doubtful, instruct the ignorant, and admonish sinners (Compendium of the Catechism of the Catholic Church, Appendix B, Formulas of Catholic Doctrine).  By these works, the Church seeks to protect people from the attraction of sin.  As the Bible states in Hebrews 3:12-13:

Take care, brethren, lest there be in any of you an evil, unbelieving heart, leading you to fall away from the living God.  But exhort one another every day, as long as it is called “today,” that none of you may be hardened by the deceitfulness of sin.

The devil presents sin as attractive and “no big deal.”  That is why it is so important for people of faith to instruct and support each other in the Truth, and to admonish those who teach sin as normal or “no big deal.”  It is not merciful to ignore a person’s sins.  It is not mercy to remain silent and implicitly accept the actions of a person who lives a lifestyle that is not in accordance with the Church’s teaching, be that living with someone before marriage, committing immoral acts with another, or supporting evils such as the abortion of the “redefinition” of marriage.  It is important for the Church, both leaders and lay faithful, to speak out against those sins.  That is not judging – that is mercy.