• St. Matthew had taught and preached in Palestine in Syriac (Aramaic)
• St. Mark taught and preached in Rome in Latin
• St. Luke taught and preached in Alexandria (Egypt) in Greek
• St. John taught and preached in Ephesus (Turkey) in Greek

i.e. Matthew wrote for his own countrymen, St. Mark wrote for the gentiles, and St. John addressed the gospel to the elders of Asia (because of their request) . What he wrote is a summary of what he taught about Jesus Christ (ref: Helps to the study of the Bible)

From the above we understand that:

• Though applicable to all, the Bible was written to specific sections of people at specific places
• Since the number of literate people were very few, the doctrines were taught mostly by the word of mouth.

Proofs from the Bible:

And they continued steadfastly in the apostles doctrine and fellowship in the breaking of bread. And in prayers? (Acts 2:42) - [some Bibles use the term teaching for doctrine. But in Authorized Versions the term used is doctrines meaning principles of belief].? The departing Lord doubtless wished to give the Apostles, spiritual instructions to prepare them to receive the Holy Spirit and also special directions for the future government of His church.? (The One Volume Bible Commentary John R Dummelow)

In Acts 1:2,3 we can see what Jesus Christ had done after the resurrection: being with them (apostles) during forty days and speaking of the things pertaining to the kingdom of God (He gave them the spiritual guidance as to the church and its structure and administration after the resurrection also)

So the apostles preached to the gentiles about Christ and taught the doctrines to the Christians, and exhorted them to stay firm to the doctrines. The apostles filled Jerusalem with this doctrine. (Acts 5:28). St. Paul talks of the doctrines they taught in Rom. 6:17, Rom. 16:17, Eph 4:14, 1 Tim 1:10, 4:6, 4:13,16, Titus 1:9, 2, Heb 13:9, 2 John 9.