Some clarification is needed. Paul wrote an Epistle, not a Gospel. An epistle is a letter written to a person or a church or a group of people, giving them general teaching or answers to specific questions.. A gospel is a historical account of the life of the Lord Jesus. Gospel means "Good news" in Old English.
There are 4 Gospel writers - Matthew, Mark, John and Luke. With the exception of Luke, all were eyewitnesses. The order of the Gospels in our printed Bible is that sanctioned by Tradition. The first three Gospels are frequently called the Synoptics (from the Greek terms syn "together," and opsis "view"). When placed side by side and brought under one view, these three Gospels present a striking resemblance and appear as one narrative. Not only are many of the same events and speeches recorded in each, but the order and manner in which they are narrated is nearly the same.
Matthew's Gospel was intended for the Jewish converts of Palestine, and was written in Aramaic, the language of the country.
Mark's Gospel was written for Gentiles, especially for Roman Gentile converts.
Luke was a native of Antioch - a city renowned for its learning - where he received his early education. From the fluency and perfection of his literary style, it is inferred that he was a Greek. Luke's Gospel is a record of Christ's life and teaching as preached by Paul.
St. John wrote his Gospel partly with a view to supplementing those of his predecessors but principally to establish on a firm basis the Divinity of Christ, even then impugned by Cerinth and other heretics. John has explicitly told us what was the object of his Gospel: "Many other signs also did Jesus in the sight of His disciples which are not written in this book. But these are written that you may believe that Jesus is the Christ, the Son of God; and that believing you may have life in his name" (20:30, 31). The particular scope of the author and the time of composition have given the fourth Gospel a character quite different from that of the Synoptics. In the first place, St. John's Gospel has little of the Synoptic material. This absence is not due to St. John's ignorance of the first three Gospels, for he knows the Synoptics and makes a clear allusion to their contents (1:26-33; 6:68; 12-27). St. John's aim is not to rewrite the Synoptics but to give the Church another Gospel, a more profound presentation of the Person and teaching of Christ.
The Galatians to whom the letter is addressed were Paul’s converts, most likely among the descendants of Celts who had invaded western and central Asia Minor in the third century B.C. and had settled in the territory around Ancyra (modern Ankara, Turkey). Paul had passed through this area on his second missionary journey (Acts 16:6) and again on his third (Acts 18:23).
What is the purpose of his letter? The new Christians whom Paul is addressing were converts from paganism (Gal 4:8–9) who were now being enticed by other missionaries to add the observances of the Jewish law, including the rite of circumcision, to the cross of Christ as a means of salvation. For, since Paul’s visit, some other interpretation of Christianity had been brought to these neophytes, probably by converts from Judaism (the name “Judaizers” is sometimes applied to them); it has specifically been suggested that they were Jewish Christians who had come from the austere Essene sect.
These interlopers insisted on the necessity of following certain precepts of the Mosaic law along with faith in Christ. They were undermining Paul’s authority also, asserting that he had not been trained by Jesus himself, that his gospel (i.e. message) did not agree with that of the original and true apostles in Jerusalem, that he had kept from his converts in Galatia the necessity of accepting circumcision and other key obligations of the Jewish law, in order more easily to win them to Christ, and that his gospel was thus not the full and authentic one held by “those of repute” in Jerusalem (Gal 2:2). His letter is intended to correct their misunderstandings.
What is the purpose of Paul's Epistle (letter) to the Romans? Like all Paul’s letters, Romans too arose out of a specific situation, when the apostle wrote from Greece, likely Corinth, between A.D. 56 and 58 (cf. Acts 20:2–3). Paul at that time was about to leave for Jerusalem with a collection of funds for the impoverished Jewish Christian believers there, taken up from his predominantly Gentile congregations (Rom 15:25–27). He planned then to travel on to Rome and to enlist support there for a mission to Spain (Rom 15:24, 28). Such a journey had long been on his mind (Rom 1:9–13; 15:23). Now, with much missionary preaching successfully accomplished in the East (Rom 15:19), he sought new opportunities in the West (Rom 15:20–21), in order to complete the divine plan of evangelization in the Roman world. Yet he recognized that the visit to Jerusalem would be hazardous (Rom 15:30–32), and we know from Acts that Paul was arrested there and came to Rome only in chains, as a prisoner (Acts 21–28, especially Acts 21:30–33 and Acts 28:14, 30–31).
The existence of a Christian community in Rome antedates Paul’s letter there. When it arose, likely within the sizable Jewish population at Rome, and how, we do not know. The Roman historian Suetonius mentions an edict of the Emperor Claudius about A.D. 49 ordering the expulsion of Jews from Rome in connection with a certain “Chrestus,” probably involving a dispute in the Jewish community over Jesus as the Messiah (“Christus”). According to Acts 18:2, Aquila and Priscilla (or Prisca, as in Rom 16:3) were among those driven out; from them, in Corinth, Paul may have learned about conditions in the church at Rome. Paul writes to introduce himself and his message to the Christians at Rome, seeking to enlist their support for the proposed mission to Spain. He therefore employs formulations likely familiar to the Christians at Rome. The gospel (i.e. message) Paul presents is meant to be a familiar one to those in Rome, even though they heard it first from other preachers.
Paul’s Letter to the Romans is a powerful exposition of the doctrine of the supremacy of Christ and of faith in Christ as the source of salvation. It is an implicit plea to the Christians at Rome, and to all Christians, to hold fast to that faith. They are to resist any pressure put on them to accept a doctrine of salvation through works of the law (see note on Rom 10:4). At the same time they are not to exaggerate Christian freedom as an abdication of responsibility for others (Rom 12:1–2) or as a repudiation of God’s law and will.
To sum up, the Gospels are the actual teachings of Jesus Christ. The Epistles (letters) are directed to specific groups of christian converts in various communities, intended to correct misunderstandings or reinforce what they have already been taught (orally preached). ALL of these "communities" or "churches" were united in one faith, i.e. the Catholic Church.
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There was no other text but The Bible "GOD's Word" and their GOD inspired books and letters the churches were required to abide by that is my point. The NT Books were all available to believers early on. It was enough and they knew to keep it simple. They didn't say here you need to study this Dogma and The Bible and that was not what was intended. This brought corruption upon the churches and opened the door for abuses. The Bible is The Word of GOD not mans additional writs done past the early Apostles and Disciples writings. Thus the necessary correction to this came in the Reformation.
The Apostles didn't set up a earthly figurehead over all the churches {no not even Peter except as the leader of the twelve} NOR were they bound to be under such rule. Example Paul was not answerable to Peter, did not need his permissions, anointment, appointment, nor answer to him and Paul clearly states this. Peter was not Paul's Pope nor anyone elses. He was simply a man who lead the twelve in Jerusalem. Actually soon the Twelve were scattered because they were not fulfilling The Great Commission rather they were living in Jerusalem in a common community and The Gospel was not going out to the world. Thus came the persecution which did indeed scatter them and form many more churches and take The Word to many places long before the church in Rome gained power.
IOW The RCC wasn't then and never has been the church of churches nor the only sanctioned church. The Word Of GOD went north, east, south, and west, not just straight to Rome and kept by a few persons as some would want all to believe.