From what is said it must also be confessed that there is but one God, not many gods. For we attribute to God supreme goodness and infinite perfection, and it is impossible that what is supreme and most perfect could be common to many. If a being lack anything that constitutes supreme perfection, it is therefore imperfect and cannot have the nature of God. The unity of God is also proved from many passages of Sacred Scripture. It is written: Hear, O Israel, the Lord our God is one Lord; again the Lord commands: Thou shalt not have strange gods before me; and further He often admonishes us by the Prophet: I am the first, and I am the last, and besides me there is no God. The Apostle also openly declares: One Lord, one faith, one baptism. (Catechism of Trent, no. 1012)

Now We consider another abundant source of the evils with which the Church is afflicted at present: indifferentism. This perverse opinion is spread on all sides by the fraud of the wicked who claim that it is possible to obtain the eternal salvation of the soul by the profession of any kind of religion, as long as morality is maintained. Surely, in so clear a matter, you will drive this deadly error far from the people committed to your care. With the admonition of the Apostle that ‘there is one God, one faith, one baptism’ (Eph 4:5) may those fear who contrive the notion that the safe harbor of salvation is open to persons of any religion whatever. They should consider the testimony of Christ Himself that ‘those who are not with Christ are against Him,’ (Lk 11:23) and that they disperse unhappily who do not gather with Him. Therefore ‘without a doubt, they will perish forever, unless they hold the Catholic faith whole and inviolate’ (Symb. s. Athan.). Let them hear Jerome who, while the Church was torn into three parts by schism, tells us that whenever someone tried to persuade him to join his group he always exclaimed: ‘He who is for the See of Peter is for me’ (epist. 57). A schismatic flatters himself falsely if he asserts that he, too, has been washed in the waters of regeneration. Indeed Augustine would reply to such a man: ‘The branch has the same form when it has been cut off from the vine; but of what profit for it is the form, if it does not live from the root?’ (In ps. contra part. Donat.) (Gregory XVI. Encyclical Mirari vos, August 15, 1832)

The holy, Catholic, Apostolic, Roman Church believes and confesses that there is one, true, living God, Creator and Lord of heaven and earth, omnipotent, eternal, immense, incomprehensible, infinite in intellect and will, and in every perfection; who, although He is one, singular, altogether simple and unchangeable spiritual substance, must be proclaimed distinct in reality and essence from the world; most blessed in Himself and of Himself, and ineffably most high above all things which are or can be conceived outside Himself (can. 1-4). (Denzinger-Hünermann 3001: Vatican Council I, Session III, Dogmatic Constitution Dei Filius, April 24, 1870)

It is essential to realistically admit, with deep and pained sentiment, that in part, Christians today feel lost, confused, perplexed and even disillusioned; ideas conflicting with the revealed and consistently taught truth have been widely spread; real heresies in dogmatic and moral fields have been promoted, creating doubts, confusions, rebellions, even the Liturgy has been manipulated; immersed in the intellectual and moral ‘relativism’, and consequently permissiveness, Christians are tempted toward atheism, agnosticism, vaguely moralistic illuminism, and a sociological Christianity, without defined dogmas and without objective morality. (John Paul II. Address to the First National Conference on Popular Missions during the 80s, February 6, 1981)

In their desire, however, to keep the moral life in a Christian context, certain moral theologians have introduced a sharp distinction, contrary to Catholic doctrine, (cf. Council of Trent, Cum Hoc Tempore: DS1569-1571) between an ethical order, which would be human in origin and of value for this world alone, and an order of salvation, for which only certain intentions and interior attitudes regarding God and neighbour would be significant. This has then led to an actual denial that there exists, in Divine Revelation, a specific and determined moral content, universally valid and permanent. The word of God would be limited to proposing an exhortation, a generic paraenesis, which the autonomous reason alone would then have the task of completing with normative directives which are truly ‘objective’, that is, adapted to the concrete historical situation. Naturally, an autonomy conceived in this way also involves the denial of a specific doctrinal competence on the part of the Church and her Magisterium with regard to particular moral norms which deal with the so-called ‘human good’. Such norms would not be part of the proper content of Revelation, and would not in themselves be relevant for salvation. No one can fail to see that such an interpretation of the autonomy of human reason involves positions incompatible with Catholic teaching. (John Paul II. Encyclical Veritatis splendor, August 6, 1993)

The Church’s constant missionary proclamation is endangered today by relativistic theories which seek to justify religious pluralism, not only de facto but also de iure (or in principle). […] On the basis of such presuppositions, which may evince different nuances, certain theological proposals are developed — at times presented as assertions, and at times as hypotheses — in which Christian revelation and the mystery of Jesus Christ and the Church lose their character of absolute truth and salvific universality, or at least shadows of doubt and uncertainty are cast upon them. […] Not infrequently it is proposed that theology should avoid the use of terms like ‘unicity’, ‘universality’, and ‘absoluteness’, which give the impression of excessive emphasis on the significance and value of the salvific event of Jesus Christ in relation to other religions. In reality, however, such language is simply being faithful to revelation, since it represents a development of the sources of the faith themselves. From the beginning, the community of believers has recognized in Jesus a salvific value such that he alone, as Son of God made man, crucified and risen, by the mission received from the Father and in the power of the Holy Spirit, bestows revelation (cf. Mt 11:27) and divine life (cf. Jn 1:12; 5:25-26; 17:2) to all humanity and to every person.
In this sense, one can and must say that Jesus Christ has a significance and a value for the human race and its history, which are unique and singular, proper to him alone, exclusive, universal, and absolute. Jesus is, in fact, the Word of God made man for the salvation of all. In expressing this consciousness of faith, the Second Vatican Council teaches: ‘The Word of God, through whom all things were made, was made flesh, so that as perfect man he could save all men and sum up all things in himself. The Lord is the goal of human history, the focal point of the desires of history and civilization, the centre of mankind, the joy of all hearts, and the fulfilment of all aspirations. It is he whom the Father raised from the dead, exalted and placed at his right hand, constituting him judge of the living and the dead’.⁴⁵ ‘It is precisely this uniqueness of Christ which gives him an absolute and universal significance whereby, while belonging to history, he remains history’s centre and goal: ‘I am the Alpha and the Omega, the first and the last, the beginning and the end’ (Rev 22:13)’ (John Paul II, Redemptoris missio, 6).
[Note 45: Second Vatican Council, Pastoral Constitution Gaudium et spes, 45. The necessary and absolute singularity of Christ in human history is well expressed by Saint Irenaeus in contemplating the preeminence of Jesus as firstborn Son: ‘In the heavens, as firstborn of the Father’s counsel, the perfect Word governs and legislates all things; on the earth, as firstborn of the Virgin, a man just and holy, reverencing God and pleasing to God, good and perfect in every way, he saves from hell all those who follow him since he is the firstborn from the dead and Author of the life of God’ (Demonstratio apostolica, 39: SC 406, 138)] (Congregation for the Doctrine of the Faith, Dominus Iesus, August 6, 2000)

Love — caritas — is an extraordinary force which leads people to opt for courageous and generous engagement in the field of justice and peace. It is a force that has its origin in God, Eternal Love and Absolute Truth. (Benedict XVI. Encyclical, Caritatis in veritate, June 29, 2009)