In the United States, a person can follow any religion he likes, or no religion. And he can call himself a Christian whether or not that is particularly true. So there are a lot of people walking around with the ‘Christian’ tag on them who do not meet the normative definition of ‘Christian.’
Some Christians say that real Christians are ones that have had a ‘born again’ experience that they remember, or that have gone forward at a ‘altar call’ in Evangelical churches that have that practice. Other Christians say that being an active Christian can start at the sacrament of baptism, even an infant baptism, and can continue as a child is raised in a Christian home where prayer and church attendance are the norm.
A nominal Christian is a Christian ‘in name only.’ Why does he take the name of Christian? For some people, claiming Christianity as a religion is just another way of saying ‘my family is not Jewish.’ If they have parents, grandparents or great-grandparents who were raised as Christians, they feel they are Christian enough— they are just not ‘fanatics’ about it.
Other people honestly think that if they believe in God and sometimes ask this God for stuff, like help in an emergency or a winning lottery ticket, that makes them Christian, unless their family was Jewish or they have taken up Buddhist meditation.
It does not help that in addition to the faithful Christians— Protestant and Catholic— who believe something that a Christian from 200 years ago would recognize as Christian, there are also very progressive Christians who make headlines. For example, some progressive Christians have blessed abortion centers and said that committing abortions is what Jesus would do. That reinforces a perception that in Christianity, anything goes and you can believe any old thing and it can be part of Christianity.
Nominal Christianity is not the same thing as progressive Christianity. Progressive Christians, as far off from the New Testament as their faith can be, are living a faith that they believe is the modern version of Christianity. Nominal Christians aren’t actively practicing any faith at all. They don’t usually know enough about Christianity to know there is something missing in their faith life.
In fiction, nominal Christians play a role in Christian fiction often by being an obstacle or a challenge to active Christians. In the ‘Left Behind’ series, the main characters included nominal Christians who became real Christians after the shock of the ‘rapture’ event.
In secular fiction, nominal Christians are often seen as sensible and non-fanatic Christians by those writers who know little. Though I’ve never read a book in which a man who doesn’t own a Koran, has never fasted for Ramadan, and who has never been to a mosque or said even one of the five daily Muslim prayers is named as a ‘non-fanatic Muslim.’ Muslims are expected to have some hints of their faith in their lives, both in fiction and in real life. Christians should have that as well. If they don’t, but still say they are Christians, we may suspect that perhaps they are nominal Christians.
Authors who know better should never present nominal Christians as ‘better’ Christians, any more than the no-mosque, no-prayer guy is a ‘better’ Muslim. Religions, both in the real world and our fictional worlds, have content. Nominal Christians, or nominal Muslims, or nominal Buddhists lack that content and so should not be representative of those faiths.