In this era of the church, the one that we are living in, we have our own problems with certain doctrinal matters. The first century also had problems with certain doctrines that they had to deal with. The very first bridge that they had to cross had to do with justification—justification by faith in the blood of Jesus Christ.
This was a new concept, which is why God commissioned the apostle Paul to write so much about justification by faith—not by works, not by earning justification, but by faith in what God said and what Jesus Christ did. This doctrinal instruction takes up much of the books of Romans and Galatians.
The second bridge they had to cross was law and grace. Even today, people like to separate the two, as if a Christian cannot believe in law and grace at the same time. The apostles had to convince the people that law and grace are not opposed to one another, but work in harmony to complete the process of justification and then sanctification. God not only forgives us, but He also gives us gifts by His Spirit by which we can be sanctified unto holiness, the middle part of the process of salvation, which absolutely cannot be left out.
The third thing is the second coming of Christ. As time passed, the pressure mounted and the return of Jesus Christ became increasingly important in the minds of people. It naturally led people to believe that they had plenty of time to overcome, and it seemed to work to cast them adrift. This is why Paul says that the Hebrews were neglecting their salvation.
In Matthew 24:42, we find that Jesus anticipated this. He really understood human nature. Incidentally, do you want to know what it is that causes people to go to sleep spiritually, so that you can be aware? It is not a hard principle at all to understand. It is having to face so many difficulties, so many pressures, that one becomes weary with facing them. This is a simplification, but it is true. When people have to face so many stresses, they become apathetic and say, "What's the use?" We need to stir ourselves up and recognize that this can happen to us.