Syncretism and indifferentism are sins against the First Commandment. Syncretism tries to combine different religions (including paganism) to create unity. Indifferentism teaches that all religions are the same and equally pleasing to God. To embrace either error ultimately annihilates truth in the one, true God. Listen to past popes who defended the integrity of the faith against these false ideas!
Pope Leo XII (Ubi Primum): A certain sect, which you surely know, has unjustly arrogated to itself the name of philosophy, and has aroused from the ashes the disorderly ranks of practically every error. Under the gentle appearance of piety and liberality this sect professes what they call tolerance or indifferentism. It preaches that not only in civil affairs...but in religion, God has given every individual a wide freedom to embrace and adopt without danger to his salvation whatever sect or opinion appeals to him on the basis of his private judgment. The Apostle Paul warns us against the impiety of these madmen. "I beseech you, brethren, to behold those who create dissensions and scandals beyond the teaching which you have learned. Keep away from such men. They do not serve Christ Our Lord but their own belly, and by sweet speeches and blessings they seduce the hearts of the innocent."
Pope Pius IX (Qui Pluribus): Also perverse is the shocking theory that it makes no difference to which religion one belongs, a theory which is greatly at variance even with reason. By means of this theory, those crafty men remove all distinction between virtue and vice, truth and error, honorable and vile action. They pretend that men can gain eternal salvation by the practice of any religion, as if there could ever be any sharing between justice and iniquity, any collaboration between light and darkness, or any agreement between Christ and Belial.
Pope St. Pius X (Notre Charge Apostolique): "But stranger still...are the audacity and frivolity of men who call themselves Catholics and dream of re-shaping society...establishing on earth, over and beyond the pale of the Catholic Church, "the reign of love and justice" with workers...of all religions and of no religion, with or without beliefs, so long as they forego what might divide them... [and] share what unites them.... When we consider the forces, knowledge, and supernatural virtues which are necessary to establish the Christian City, and the sufferings of millions of Martyrs, and the light given by the Fathers and Doctors of the Church, and the self-sacrifice of all the heroes of charity, and a powerful hierarchy ordained in Heaven, and the streams of Divine Grace...impregnated by the life and spirit of Jesus Christ, the Wisdom of God, the Word made man...it is frightening to behold new apostles [spreading] seductive confusion [with] the words Liberty, Justice, Fraternity, Love, Equality, and human exultation.