ECFs do appear to point to (proto)catholic ideas and attitudes, but that happens simply because they have been selected precisely through the criterion of their being (proto)catholic.
But then,who were the early Church fathers that were* not* approved by the Catholic Church? Should we consider Marcion,Valentinus,Arius,or Pelagius to be Church fathers?
The reason why the belief that there was no salvation outside of the Catholic Church came about is because there were,from the beginning,heretics (John calls them anti-christs) preaching false doctrines and leading people away from the teachings of the apostles and of the apostolic Church.
There was only one true Church and it was always called Catholic. There was no entity called the Christian Church or Church of Christ or Biblical Church. There was no entity called the Orthodox Church either,the faith was called orthodox.
"The Article, In one Holy Catholic Church,’ on which, though one might say many things, we will speak but briefly. It is called Catholic then because it extends over all the world, from one end of the earth to the other; and because it teaches universally and completely one and all the doctrines which ought to come to men’s knowledge, concerning things both visible and invisible, heavenly and earthly… for this cause the Faith has securely delivered to thee now the Article, And in one Holy Catholic Church;’ that thou mayest avoid their wretched meetings, and ever abide with the Holy Church Catholic in which thou wast regenerated. And if ever thou art sojourning in cities, inquire not simply where the Lord’s House is (for the other sects of the profane also attempt to call their own dens houses of the Lord), nor merely where the Church is, but where is the Catholic Church. For this is the peculiar name of this Holy Church, the mother of us all, which is the spouse of our Lord Jesus Christ, the Only-begotten Son of God.” Cyril of Jerusalem, Catechetical Lectures, 18:23,26 (A.D. 350).
< Now, this point seems to rest on the presupposition that in later centuries ( when ?) there was a scrupolous selection among a wide pool of early christian writers, and, because of their catholic (catholic/orthodox) credentials, a limited part of them have been promoted to Fathers. >
The writings of Church fathers such as Irenaeus,Basil,Cyril of Alexandria,Athanasius,Jerome,Ambrose,Augustine were recognized by the Roman Catholic Church as orthodox even during their own lifetime or shortly after they died. It is not a matter of their being officially accepted as Church fathers at a certain period much later in time. They are called fathers because they were teachers,and their teachings were well known even in their own lifetime.
< The first question about which is: where can we find this large set of authors, among whom the ECFs had been selected ? >
There are the early Church fathers,and there are the heretics and schismatics who were condemned as such even during their own lifetime. In the case of Origen,he is a Church father even though some of his ideas are heretical and were condemned as such by other Church fathers after he died. Tertullian also is a Church father,but he turned Donatist in the last few years of his life and broke communion with Rome. So Origen and Tertullian were not made saints like the other famous Church fathers.
< This is not to mean that there were not early christian writers with non-catholic or anti-catholic attitudes, mirroring the existence of dissents present ab initio, and that a lesser likelyhood of survival to our days of their works has to be considered ( since they were not on the winning side) . >
There is good reason why they were not on the winning side,if you consider what they believed and taught and how they undermine the fullness of the gospel and the spirit of charity.
It always has something to do with pride – the refusal to abide within the bounds of the apostolic teaching of the Church,and within “catholic” communion.
“Inasmuch, I repeat, as this is the case, we believe also in the Holy Church, [intending thereby] assuredly the Catholic. For both heretics and schismatics style their congregations churches. But heretics, in holding false opinions regarding God, do injury to the faith itself; while schismatics, on the other hand, in wicked separations break off from brotherly charity, although they may believe just what we believe. Wherefore neither do the heretics belong to the Church catholic, which loves God; nor do the schismatics form a part of the same.” Augustine, On Faith and Creed, 10:21 (A.D. 393).