Apostle Paul stayed in Ephesus for three years, preaching night and day (Acts 20:31). That means that the Ephesian Christians listened to many hundreds of sermons from Paul's lips. They had seen extraordinary miracles wrought by the Lord in their midst (Acts 19:11). From their midst, the word of God had spread to all the surrounding parts of Asia Minor during a short period of two years. They had experienced revival (Acts 19:10, 19). They were the most privileged of all the churches in apostolic times. They were also undoubtedly the most spiritual church in Asia Minor at that time. (We can see that from Paul's letter to the Ephesians, where he had to correct no error in their midst, unlike the way he had to, in the other churches to which he wrote). But when Paul was leaving Ephesus, he warned the elders there that things would take a turn for the worse in the next generation, under the new leadership of the church. He told them that savage wolves would come into their midst and that from among their own midst would arise men speaking perverse things, drawing people after themselves, instead of drawing people to the Lord (Acts 20:29, 30).
As long as Paul was there, no wolf had dared to enter the flock at Ephesus. Paul was a faithful doorkeeper (See Mark 13:34), who had spiritual authority from the Lord, because he was anointed, because he feared God and because he sought the Lord's interests and not his own. But he also had enough spiritual discernment to know that the spiritual condition of the elders in Ephesus was bad - and so he knew that things would deteriorate once they took over the leadership of the church. Paul did not give the elders a prophecy of what would definitely happen at Ephesus. No. It was only a warning. It did not have to happen like he predicted - if the elders would only judge themselves and repent.
Jonah once prophesied destruction on Nineveh. But it did not happen as he predicted, because the people of Nineveh repented. The church at Ephesus also could have escaped the fate that Paul predicted. But alas, the new generation of leaders in Ephesus never took Paul's warning seriously and drifted away from the Lord. By the end of the first century, the third generation had come into power. And then things became really bad. Their doctrines were still correct and they were zealous in Christian activity. They probably still had their all-night prayer meetings and their other special meetings. But their spiritual state was so bad that the Lord was about to remove His recognition of them as a church. What was their crime? They had lost their devotion to the Lord (Revelation 2:4, 5).
What does the history of the church at Ephesus teach us? Just this - that no doctrine is as important as a fervent devotion to the Lord Himself. There is one- and only-one-mark of true spirituality - that the life of Jesus is manifested increasingly in our behaviour and this in turn can come only by an increasing personal devotion to the Lord Himself.
Paul was a godly man - a fervent and faithful apostle who was devoted to the Lord Jesus until the very end of his life. And he warned believers everywhere that Satan would try every means possible to turn them away from "simple devotion to Christ" (2 Corinthians 11:3). Errors in doctrinal matters such as "baptism in water" and "baptism in the Holy Spirit", are not at all as dangerous as losing one's personal devotion to Christ. Yet many believers never seem to realise this. We see that even Paul could serve God's purpose only in his own generation. Those who lived with him like Timothy, imbibed his spirit and lived in selfless devotion to Christ (Philippians 2:19-21). But otherwise, Paul could not transmit his spirituality even to the second generation of believers in the churches he had founded.