Forgive me; perhaps all this is superfluous. At this point, though, I would like to say a little about the fact that at the present time a significant part of the Christian world tends to accept one of the most dangerous heresies. What is consists of is people saying that in our days there is not one Church which has kept fully the true teaching of Christ; or which possesses complete knowledge of the mystery of the holy, grace-filled Christian life on the ethical and ascetic level. Supposedly, many of the Churches which are nominally Christian have equal grace, and because of that we should proceed towards the union of the Churches on the basis of some common program. One of the most frequent questions which one comes across is the question of who will be saved and who will not be saved. These people usually think that it is not only the Orthodox who will be saved (according to Orthodox teaching), not only the Catholics (according to Catholic teaching), but all virtuous people in general who believe in Christ. This viewpoint has passed from the Protestants to the faithful of other Churches. There are many among the Orthodox who hold this opinion.
Some people think that no single one of the existing Churches can receive the fulness of knowledge and grace, because each one of them in one or another degree has deviated from the truth. They think that only now ‘at the end of the ages’ they (these sages) have fully grasped the spirit of the teaching of Christ, and that the entire Christian world has been led astray for many centuries until now. That now the time has come when we must unite all the separated parts into one universal and apostle Church, which will have the fulness of truth in all its aspects, even though this union will only embrace what is common to all the Churches. What is even worse, some of them are pondering in their hearts a certain high, supra-ecclesial, mystical, understanding of Christian religion, which… I won’t say more about this.
I digressed into discussing this for one reason only: to tell you that I very much want you (and I pray to God for this) not to be deceived by all that, but to be convinced firmly in your heart and mind that on this earth there is one unique and true Church which Christ founded; that this Church maintains unspoiled the teaching of Christ, that she in her totality (and not in her individual members) possesses the fulness of knowledge and grace and infallibility. [I want you to be convinced] that what for several people seems to be incompleteness in her teaching is none other than the potential for some scholarly elaboration of her inexhaustible and infinite riches — this, however, does not contradict in any measure what I said above about her possessing the fulness of knowledge.
The definitive form of expression of the Church’s teaching at the Ecumenical Councils cannot be subjected to any change. All future academic work must obligatorily concur with what was given in divine revelation and in the teaching of the Ecumenical Councils of the Church. The same is true in connection with grace: only the one and unique Church can have the fulness of grace. All the other Churches, however, do have grace because of their faith in Christ, but not in its fulness. We can, furthermore, believe that in our days there are still people who, by the grace of the Holy Spirit, are equal to the great Saints of the Church of ancient times. (I am saying this in connection with what I heard about several people in Russia.) [This is] because Christ is the same yesterday and today and forever (Heb. 13:8). All this is the truth. Whoever departs from this faith will not stand. (Striving for Knowledge of God pp. 144-146. Letter 11)