The main danger confronting the pulpit in this matter is to assume that all who claim to be Christians, and who think they are Christians, and who are members of the Church, are therefore of necessity Christians. … This is dangerous and wrong for this reason, that if you assume that, you will tend therefore, in all your services, to preach in a manner suited to Christian believers. Your messages will always be instructional, and the evangelistic element and note will be neglected, perhaps, almost entirely. 

This is a very great and grievous fallacy.  Let me give you reasons for saying that. I would start with my own personal experience.  For many years I thought I was a Christian when in fact I was not.  It was only later that I came to see that I had never been a Christian and became one.  But I was a member of a church and attended my church and its services regularly.  So anybody assuming, as most preachers did, that I was a Christian was making a false assumption.  It was not a true assessment of my condition. [I was received into the Church because I could give the right answers to various set questions; but I was never questioned or examined in an experimental sense.]  What I needed was preaching that would convict me of sin an make me see my need, and bring me to true repentance and tell me something about regeneration. But I never heard that. The preaching we had was always based on the assumption that we were all Christians, that we would not have been there in the congregation unless we were Christians. This, I think, has been one of the cardinal errors of the Church especially in this present century.

But this has been reinforced many times in my experience as a preacher and as a pastor. I think I can say quite accurately that my most common experience in conversation with people who have come to me in my vestry to discuss the questions of becoming members of the church, has been this. I have questioned them as to why they want to become members, and what their experience is and so on. The commonest answer I have had, particularly in London for over thirty years, has been something like this. These people–and quite frequently they were either undergraduates or young graduates–would tell me that they came up to London to the University from their home churches fully believing that they were Christians. They had no doubt about that, and either they had asked their home church, before coming up to London, where they should go on Sundays, or else they had been referred to us by their home church. They went on to tell me that having come in that way, and having listened to the preaching, and especially on Sunday nights, when, as I have already said my preaching was invariably evangelistic, the first thing they discovered was that they had never been Christians at all and that they were living on a false assumption.  At first, some of them were honest enough to confess, they had been rather annoyed at this.  They did not like it, and they had resented it; but that was the fact.  Then, realizing that though they did not like it, that this was the truth, they continued to come. This might go on perhaps for months, and they would pass through a period of repentance in great trouble about their souls. They were afraid to trust almost anything became, having previously assumed wrongly that they were Christians, they were now afraid of repeating the same error. Then eventually they had come to see the truth clearly and had experienced its power and become truly Christian. 

That has been my commonest experience in the ministry. It shows the complete and dangerous fallacy of assuming that anybody who comes to a service regularly must be a Christian.

From D. Martyn Lloyd-Jones, Preaching and Preachers, 1971, pp. 146, 147