Since you mentioned Mere Christianity...

One of the things I am always struck by when I read authors such as C.S. Lewis (and Francis Schafer and GK Chesterton) is their clarity of vision. When reading their books you would almost think that they were writing directly to address the issues of our current day.

These men had the forsight to not only recognize the problems of their own day, but to see what those problems would lead to in the future.

In Mere Christianity, one of the things Lewis addresses is that Christianity ultimately MUST be defined as a set of beliefs. I must be creedal in nature. He saw in his own day that people were beginning to define "christian" to mean "good person" or "nice guy" and that this was a danger to the Christian faith.
The problem with defining christianity in those terms is that it then becomes impolite, or even mean to say that someone is not a christian. If you allow "christian" to be defined as "likable" or "nice guy" or "good person" it then becomes a personal insult to say that someone is not a christian.

This, in turn, poses a problem because christianity is constantly under seige by false teachings and deception. If we are going to guard the truth of our faith we must be able to say "this man and what he is teaching are not christian". However we can not say that because we have allowed the word christian to be redefined. Now it has become a personal attack to say something like that. The result is that there is no way to preserve orthodoxy.

Lewis made the point that to say someone is a christian does not mean they are good, nor does it mean they are close to God. It simply means they claim to believe a specific set of doctrines. If a person does not follow in life what they claim to beleive, they are a bad christian. If a person claims to believe something that is not christian, but yet follows good morality, they may be a nice person, they may even be closer to God than someone who is a bad christian... but they are not a christian.

This has become a significant problem in many churches. People who do not believe the core doctrines of the christian faith are permitted to work within the church, to become leaders in the church, and the result is that the church is corrupted, many people are lead astray, and strife and division is propagated in the church

It was for exactly this reason that the early church held councils to protect and establish the core necessary doctrines of christianity.

There is a well known legal technique generally used by defense lawyers to prevent juries from discovering the truth, often referred to as "muddying the waters". The defense will bring up as many alternate theories of who committed the crime, or how the crime could have been committed as possible. The theories don't even have to be very credible, there just has to be alot of them. The result is that the jurors can no longer clearly see the clear facts of the prosecurtion's case. They become bogged down in little details from the flood of varying theories and soon what was once clear as day, is being second guessed and doubted.
Our enemy has taken this tactic against the church. He has flooded the church with varient doctrines and theories and teachings on every issue. The result is that we lose our surity of the truth, we begin to doubt.

This is one of the reasons it is so valuable to go back and read the early writings of the church. The only way to counter a flood of counterfiets is by keeping your eyes on the truth. If you begin to look at the counterfiets, even to discern them... you will soon find yourself in muddy water. If however you keep your gaze focused on the truth, you will recognize anything that doesn't match that standard.