Paul wrote to the Colossians that the gospel was growing and bearing fruit since the day they heard it and understood the grace of God in all its truth (Col. 1:6). Thus Christians in the apostolic age did have a complete understanding of the doctrine of God’s sovereign and saving grace. But yet these same Christians were stringently warned against the deception of false teaching.

At Thessalonica there were Christians whom Paul commended, stating that they had proven their election by their hearts and works: “ For we know, brothers loved by God, that he has chosen you, because our gospel came to you not simply with words, but also with power, with the Holy Spirit and with deep conviction…You became imitators of us and of the Lord; in spite of severe suffering, you welcomed the message with the joy given by the Holy Spirit. And so you became a model to all the believers in Macedonia and Achaia. The Lord's message rang out from you not only in Macedonia and Achaia—your faith in God has become known everywhere. Therefore we do not need to say anything about it, for they themselves report what kind of reception you gave us. They tell how you turned to God from idols to serve the living and true God, and to wait for his Son from heaven, whom he raised from the dead—Jesus, who rescues us from the coming wrath.”

Yet these same Christians at Thessalonica were so confused about the second coming of Christ that they had given up their jobs and become idle convinced that the Lord’s coming was imminent. They had become busybodies and were subject to church discipline (1 Thess. 4:11-5:11, 2 Thess. 3:6-15). Apparently there were some who were either greatly tempted or had already succumbed to sexual sins (1 Thess. 4:1-8).

The point is simply that no matter how well instructed Christians are, no matter how excellent and fruitful they have been in their evangelism and good works, they still need the admonition of the word to avoid personal sin and doctrinal error.