1 All who are under the yoke as slaves are to regard their own masters as worthy of all honor so that the name of God and our doctrine will not be spoken against. 2 Those who have believers as their masters must not be disrespectful to them because they are brethren, but must serve them all the more, because those who partake of the benefit are believers and beloved. Teach and preach these principles. 3 If anyone advocates a different doctrine and does not agree with sound words, those of our Lord Jesus Christ, and with the doctrine conforming to godliness, 4 he is conceited and understands nothing; but he has a morbid interest in controversial questions and disputes about words, out of which arise envy, strife, abusive language, evil suspicions, 5 and constant friction between men of depraved mind and deprived of the truth, who suppose that godliness is a means of gain. 1 Timothy 6:1-5 (NASB)
37 If anyone thinks he is a prophet or spiritual, let him recognize that the things which I write to you are the Lord’s commandment. 38 But if anyone does not recognize this, he is not recognized. 1 Corinthians 14:37-38
20 But you have an anointing from the Holy One, and you all know. 21 I have not written to you because you do not know the truth, but because you do know it, and because no lie is of the truth. 1 John 2:20-21
Second, evangelism was a much more thorough process in New Testament times. It was not a simple rendering of the “Four Spiritual Laws,” but a much more comprehensive exposition of the Scriptures (as you can see not only in Paul’s synagogue sermons, but also in his preaching to Gentiles). It was expected that they would know the fundamentals (like the deity and humanity of Jesus, the Trinity, substitutionary atonement, baptism, etc. – see Hebrews 6:1-2). Thus, Paul could say to the Ephesian saints,
17 So this I say, and affirm together with the Lord, that you walk no longer just as the Gentiles also walk, in the futility of their mind, 18 being darkened in their understanding, excluded from the life of God because of the ignorance that is in them, because of the hardness of their heart; 19 and they, having become callous, have given themselves over to sensuality for the practice of every kind of impurity with greediness. 20 But you did not learn Christ in this way, 21 if indeed you have heard Him and have been taught in Him, just as truth is in Jesus, 22 that, in reference to your former manner of life, you lay aside the old self, which is being corrupted in accordance with the lusts of deceit, 23 and that you be renewed in the spirit of your mind, 24 and put on the new self, which in the likeness of God has been created in righteousness and holiness of the truth. 25 Therefore, laying aside falsehood, SPEAK TRUTH EACH ONE of you WITH HIS NEIGHBOR, for we are members of one another. Ephesians 4:17-25
6 I am amazed that you are so quickly deserting Him who called you by the grace of Christ, for a different gospel; 7 which is really not another; only there are some who are disturbing you and want to distort the gospel of Christ. 8 But even if we, or an angel from heaven, should preach to you a gospel contrary to what we have preached to you, he is to be accursed! 9 As we have said before, so I say again now, if any man is preaching to you a gospel contrary to what you received, he is to be accursed! Galatians 1:6-9
With reference to these texts in Ephesians and Galatians, it was assumed that even before these epistles reached the church the believers were grounded in the truth of the gospel, and could thus be warned against departing from it.
Third, even before the New Testament Scriptures were a part of the canon of Scripture, Paul and the other apostles had taught the church elders essential doctrine thoroughly.
17 From Miletus he sent to Ephesus and called to him the elders of the church. 18 And when they had come to him, he said to them, “You yourselves know, from the first day that I set foot in Asia, how I was with you the whole time, 19 serving the Lord with all humility and with tears and with trials which came upon me through the plots of the Jews; 20 how I did not shrink from declaring to you anything that was profitable, and teaching you publicly and from house to house, 21 solemnly testifying to both Jews and Greeks of repentance toward God and faith in our Lord Jesus Christ. Acts 20:17-21
26 “Therefore, I testify to you this day that I am innocent of the blood of all men. 27 “For I did not shrink from declaring to you the whole purpose of God. 28 “Be on guard for yourselves and for all the flock, among which the Holy Spirit has made you overseers, to shepherd the church of God which He purchased with His own blood. Acts 20:26-28
Fourth, it seems fairly clear to me that the things which the apostles wrote down in their epistles were the very same things that they had been teaching orally, so that what is written is a reminder, more than a new revelation (at least so far as the gospel and its fundamentals are concerned).
14 And concerning you, my brethren, I myself also am convinced that you yourselves are full of goodness, filled with all knowledge and able also to admonish one another. 15 But I have written very boldly to you on some points so as to remind you again, because of the grace that was given me from God, 16 to be a minister of Christ Jesus to the Gentiles, ministering as a priest the gospel of God, so that my offering of the Gentiles may become acceptable, sanctified by the Holy Spirit. Romans 15:14-16
17 For this reason I have sent to you Timothy, who is my beloved and faithful child in the Lord, and he will remind you of my ways which are in Christ, just as I teach everywhere in every church. 1 Corinthians 4:17
12 Therefore, I will always be ready to remind you of these things, even though you already know them, and have been established in the truth which is present with you. 13 I consider it right, as long as I am in this earthly dwelling, to stir you up by way of reminder, 14 knowing that the laying aside of my earthly dwelling is imminent, as also our Lord Jesus Christ has made clear to me. 15 And I will also be diligent that at any time after my departure you will be able to call these things to mind. 2 Peter 1:12-15
5 Now I desire to remind you, though you know all things once for all, that the Lord, after saving a people out of the land of Egypt, subsequently destroyed those who did not believe. Jude 1:5
Fifth, the New Testament church had prophets, who could proclaim the truths of the faith.
11 And He gave some as apostles, and some as prophets, and some as evangelists, and some as pastors and teachers, 12 for the equipping of the saints for the work of service, to the building up of the body of Christ; Ephesians 4:11-12
Sixth, the New Testament church leaders (and the other saints) had the Holy Spirit.
26 “But the Helper, the Holy Spirit, whom the Father will send in My name, He will teach you all things, and bring to your remembrance all that I said to you. John 14:26
7 “But I tell you the truth, it is to your advantage that I go away; for if I do not go away, the Helper will not come to you; but if I go, I will send Him to you. 8 “And He, when He comes, will convict the world concerning sin and righteousness and judgment; 9 concerning sin, because they do not believe in Me; 10 and concerning righteousness, because I go to the Father and you no longer see Me; 11 and concerning judgment, because the ruler of this world has been judged. 12 “I have many more things to say to you, but you cannot bear them now. 13 “But when He, the Spirit of truth, comes, He will guide you into all the truth; for He will not speak on His own initiative, but whatever He hears, He will speak; and He will disclose to you what is to come. 14 “He will glorify Me, for He will take of Mine and will disclose it to you. 15 “All things that the Father has are Mine; therefore I said that He takes of Mine and will disclose it to you. John 16:7-15
Suffice it to say that the New Testament seems clear that the fundamental elements of doctrine were well and widely known in the early church, so that doctrinal error should have been readily evident.