Christ has told us, warned us. It is recorded in Scripture.
Throughout time Christian people need to be vigilant. In the 19th century we Baptists had a Baptist Union in this country that was extremely large and included Baptists all over the land. There were many more Baptist churches in the country in those days, than there are now, and they all believed the word. Most of them not only believe the word, but most of them were Calvinistic Baptists, who believed in gospel preaching, the free offer of salvation, and the doctrines of grace together. And through most of the 19th century the Baptist Union was a pretty sound body. Yes, it had its problems, but towards the end of the 19th century – the 1870s, the 1880s – biblical criticism and theological liberalism started to come into the union, where the inspiration and infallibility of the Bible was no longer believed. It came first of all into some of the seminaries, and then it infected the ministry and ministers and teachers were coming into the denomination who were denying essentials. Many ministers towards the end of the century didn't believe in penal substitution: that Christ suffered and died an atoning substitutionary death, bearing away with the punishment and the guilt of his people. They said, ‘God wouldn't be so cruel as to so afflict his Son.’ The heart of the gospel was taken away and thrown away. Some people protested. Famously CH Spurgeon and his church came out of the Baptist Union in 1887, and a whole string of others did too. But the majority of people, even many of Spurgeon students, couldn't see it. ‘This is our denomination. People believe and love the word. Yes, there are some people who we must oppose, but by and large they are sound.’ And the union wouldn't expel error, and so it was in trouble. As time went by, liberalism took over most of it. Now there are very few true evangelicals left in the Baptist Union. Now it is by and large a liberal denomination, and it's going the way of all liberal denominations: women ministers, and everything. No longer does it stand where it once stood. But when I was a youngster and saved, and in a Baptist Union church as a teenager, I remember that there the secretary – and it was quite a sound church – would get up and say, ‘Oh, we are sending messengers to our beloved denomination’s annual assembly.’ They knew there were troubles, but they wouldn't face it. They didn't want difficulty. They didn't want to lose the romance of imagined soundness, so they didn't watch. They were not vigilant. They didn't protect the denominations and the faith. They didn't contend with the deathly errors that were coming in. That is what these verses are about. Watch, says the Lord. Watch; be alert; be vigilant always. Error stalks; the devil is attacking constantly.