The following is taken from the encyclical Satis Cognitum by Pope Leo XIII. The implications about the errors of modern "ecumenism" and the relationship of the Church to non-Catholic religions should be quite obvious here.
...Christ instituted in the Church a living, authoritative and permanent Magisterium, which by His own power He strengthened, by the Spirit of truth He taught, and by miracles confirmed. He willed and ordered, under the gravest penalties, that its teachings should be received as if they were His own. As often, therefore, as it is declared on the authority of this teaching that this or that is contained in the deposit of divine revelation, it must be believed by every one as true. If it could in any way be false, an evident contradiction follows; for then God Himself would be the author of error in man. "Lord, if we be in error, we are being deceived by Thee" (Richardus de S. Victore, De Trin., lib. i., cap. 2). In this wise, all cause for doubting being removed, can it be lawful for anyone to reject any one of those truths without by the very fact falling into heresy?-without separating himself from the Church?-without repudiating in one sweeping act the whole of Christian teaching? For such is the nature of faith that nothing can be more absurd than to accept some things and reject others. Faith, as the Church teaches, is "that supernatural virtue by which, through the help of God and through the assistance of His grace, we believe what he has revealed to be true, not on account of the intrinsic truth perceived by the natural light of reason, but because of the authority of God Himself, the Revealer, who can neither deceive nor be deceived" (Conc. Vat., Sess. iii., cap. 3). If then it be certain that anything is revealed by God, and this is not believed, then nothing whatever is believed by divine Faith: for what the Apostle St. James judges to be the effect of a moral deliquency, the same is to be said of an erroneous opinion in the matter of faith. "Whosoever shall offend in one point, is become guilty of all" (Ep. James ii., 10). Nay, it applies with greater force to an erroneous opinion. For it can be said with less truth that every law is violated by one who commits a single sin, since it may be that he only virtually despises the majesty of God the Legislator. But he who dissents even in one point from divinely revealed truth absolutely rejects all faith, since he thereby refuses to honour God as the supreme truth and the formal motive of faith. "In many things they are with me, in a few things not with me; but in those few things in which they are not with me the many things in which they are will not profit them" (S. Augustinus in Psal. liv., n. 19). And this indeed most deservedly; for they, who take from Christian doctrine what they please, lean on their own judgments, not on faith; and not "bringing into captivity every understanding unto the obedience of Christ" (2 Cor. x., 5), they more truly obey themselves than God. "You, who believe what you like, believe yourselves rather than the gospel" (S. Augustinus, lib. xvii., Contra Faustum Manichaeum, cap. 3).