Dr Kendall Harmon has Dr Martyn Lloyd Jones’ story:
It was my pleasure and privilege to preach for nine Sundays in Canada, in Toronto, in 1932. I well remember being welcomed on the first Sunday morning by the minister of the church who, though on vacation, was still not out of town. He introduced me, and in responding to the welcome I thought it would be wise for me to indicate to the congregation my method as a preacher. I told the congregation that my method was to assume generally on Sunday morning that I was speaking to believers, to the saints, and that I would try to edify them; but that at night I would be preaching on the assumption that I was speaking to non-Christians as undoubtedly there would be many such there. In a sense I just said that in passing. We went through that morning service, and at the close the minister asked if I would stand at the door with him to shake hands with people as they went out. I did so. We had shaken hands with a number of people when he suddenly whispered to me saying, ‘You see that old lady who is coming along slowly. She-is the most important member of this church. She is a very wealthy woman and the greatest supporter of the work.’ He was, in other words, telling me to exercise what little charm I might possess to the maximum. I need not explain any further! Well, the old lady came along and we spoke to her, and I shall never forget what happened. It taught me a great lesson which I have never forgotten.
The old lady said, ‘Did I understand you to say that in the evening you would preach on the assumption that the people ljstening are not Christians and in the morning on the assumption that they are Christians?’ ‘Yes,’ I said. ‘Well,’ she said, ‘having heard you this morning I have decided to come tonight.’ She had never been known to attend the evening service; never. She only attended in the morning. She said, ‘I am coming tonight.’ I cannot describe the embarrassment of the situation. I sensed that the minister standing by my side felt that I was ruining his ministry and bitterly regretted inviting me to occupy his pulpit! But the fact was that the old lady did come that Sunday night, and every Sunday night while I was there. I met her in her house in private conversation and found that she was most unhappy about her spiritual condition, that she did not know where she stood. She was a fine and most generous character, living an exemplary life. Everybody assumed-not only the minister but everybody else-that she was an exceptionally fine Christian; but she was not a Christian. This idea that because people are members of the church and attend regularly that they must be Christian is one of the most fatal assumptions, and I suggest that it mainly accounts for the state of the Church today.
–Martyn Lloyd Jones, Preaching and Preachers (Grand Rapids: Eerdmans, 1972), pp.147-149 (emphasis mine)