My mother died on 7 August 1995. I didn't realise, that day, my life had changed. In fact I would say it is only in the past two years, and only through my writing, that I have realised just how profound the change has been.

My mother died, as she had lived, unselfishly. After she'd died, my wife Susan and I were just in time for Sunday lunch at my aunt's. That may sound frivolous, but it was so typical of her I actually believe that some unconscious influence was at work.

I had never seen a person die before. Well, I'd seen it often in films and on television, of course. Grasping relatives, an ugly death rattle, an unfinished sentence of great importance. "You'll find the map of the island under the… aaargh!" My mother's death was not like that. It was, quite simply, beautiful. It was serene.

She had lived about as happily as it was possible to live in the 20th century, for almost 95 years. She had been ill and in hospital only for the last two weeks. At times, during those two weeks, she had been restless and disturbed, but that Sunday morning she became more and more peaceful. Her breathing began to get slower. She had worried for Wales, and I had no doubt this contributed heavily to her worry lines, but now all those lines disappeared – her face became smooth and she looked young again. Her breathing faded and slowed so imperceptibly it was hard to recognise the moment she actually died.

My father's death, more than 30 years before, had been very different. He had retired as senior maths master and assistant headmaster of the City of London School, a dinner was given in his honour, he made a very good speech, warmly applauded, and he sat down and died, too soon of course, but with the evidence of his popularity ringing in his ears. I had to identify the body the next day, and he too looked at peace, but because I wasn't present at his death it all had a less profound effect upon me.

But I saw my mother die. I saw her slip away. I saw that it was time. I saw that it was seemly. I can honestly say, on reflection, that witnessing her death took away from me all fear of my death. (Not of my wife's death. I fear loss dreadfully.)

That doesn't mean I welcome the ravages of old age. I fight against them. In my 70s I have taken on a fitness trainer and last month I began to tweet! I hope that I will not die in great pain or in an old people's home. But I no longer fear the moment when I willcease to exist, I no longer resent my mortality, I no longer worry about the brevity of life. And I can trace that feeling back to that Sunday in that hospital in Colchester, when I saw how much more peaceful than birth death can be, and how natural it is to die.

But this is only a part of what happened to me that Sunday in 1995. I was confirmed in the Church of England at school, and for a while took my religion very seriously. I come from nonconformist and teetotal stock. People who know me will be astounded to learn that at the age of 17, with a school cricket team, I refused to go into a pub as I thought they were wicked places. I know better now. I know that they are wicked places, and I have been into thousands of them. In fact, the following year, at the age of 18, I led the team into the pub.

My belief waned and, at some unrecognised moment, died. I always regretted that my father didn't live to see my occasional successes – less, I hope, for my own sake than for his, because he would have been so proud. However, I did catch myself looking up into the sky occasionally and wondering if he did exist in some shape or form or sense, and could laugh at The Fall and Rise of Reginald Perrin, wherever or whatever he was.

But when my mother died I knew I would never see her again in any shape or form. I wasn't thinking of it at the time, of course – there were many more practical things to see to – but I realised it later, as I reflected on that remarkable morning. I used the word "knew" there: "I knew that I would never see her again." I mean that I had absolute certainty. It doesn't necessarily mean that I was right. But when religious people say that they know that there is a God, they are praised for the strength of their faith. When a disbeliever says he knows that there isn't, he is accused of arrogance. The playing field is not level.

I could make all sort of points against religion. I see no sign of an overall purpose in our existence, I believe that many of the world's ills have been caused by religion, I think the Catholic opposition to birth control is helping to put impossible strain on our planet's resources, I am appalled by the widespread evidence of paedophilia in priests. This is going to sound dreadful, but I am appalled at the stupidity of some people's faith. Television showed a man who had miraculously escaped injury when a church collapsed during a tornado in the US. He thanked God, not seeing that if God had saved him he could have saved the many people who died in the tragedy.

But the most important thing that happened to me in the wake of my mother's death wasn't the strengthening of my feelings against religion. It was the strengthening of my feelings for disbelief. I believe that there are just as many of the "Christian virtues" to be found among the faithless as the faithful. Furthermore, these qualities are explored and developed along individual paths. We have no God whom we can burden with the responsibility for our actions.

Loss of faith. It sounds so negative. I didn't lose faith. I gained faith. Faith in people. I am proud to describe myself as a humanist. Last year I joined the British Humanist Association, and I don't think I would have made this move if I had not seen my mother die that sunny Sunday morning.

This growing conviction has had quite an effect on my writing – on the novels, at least. I am sometimes described as a comic novelist, but I describe myself simply as a novelist. I write about life, and in life I see much humour and much tragedy, and that is what I write about. I don't set out to preach and I would be upset if my novels were described as didactic. Nevertheless, I can say without doubt that my new novel, Obstacles to Young Love, and the next one, Life After Deborah, which I have almost completed, are books which I would not have written had I not witnessed my mother's death. I would describe them as being humanist books as well as humorous ones. I would like them to give thought to the faithful and encouragement to the faithless.

So I think I can truly describe the day my mother died as a day which changed my work as well as my life – for the time being, at any rate. My feeling is that I have explored this theme meaningfully and largely to my satisfaction in these two books and now I must move on, but time will tell.

I respect people's faith, but I am actually quite horrified by missionary work. I regard it as, essentially, arrogant. I wouldn't dream of going around trying to persuade people not to believe in God. I don't think I have the right, and I don't think society would believe that I had the right. But many religious people do believe that they have the right. It is not, I say again, a level playing field.

I find it hard to believe now that at school we had compulsory chapel every day. I dislike the whole concept of faith schools, not just because they lead to the absurd situation of parents who don't believe or barely believe attending church purely in order that their children may qualify for admission, but because I don't believe secular organisations should be taking official sides on this increasingly important question of whether there is a God.

An irony of all this is that if my mother could hear me, could read this, she would be very distressed and would be horrified to think that her death had led me down this road. Well, there it is, it's what has happened and luckily I believe (know?) that she can't.

David Nobbs is best known as the creator of Reginald Perrin. His latest novel is Obstacles to Young Love (Harpercollins, £7.99)