Jesus’s close friend and disciple, John, writes the first Johannine epistle to Christians in (probably) Ephesus, where he is understood to have eventually lived.
In 1 John 1, the disciple describes God as light, with no darkness in him at all. But, he warns (CSB):
If we say, “We have fellowship with him,” and yet we walk in darkness, we are lying and are not practicing the truth. If we walk in the light as he himself is in the light, we have fellowship with one another, and the blood of Jesus his Son cleanses us from all sin.
In modern Western society, the idea of being a nominal Christian is less attractive than it once was. But some people do still assert that they are Christians and they say they will go to heaven when they die. But their Christianity is like seed sown on among thorns. As Jesus said (Matthew 13 CSB):
Now the one sown among the thorns—this is one who hears the word, but the worries of this age and the deceitfulness of wealth choke the word, and it becomes unfruitful.
I don’t like the idea of saying pointing at other denominations or traditions and saying that they aren’t real Christians. In times past, Protestants used to say that about Catholics. Fortunately, that’s gone out of fashion. But it is certainly true that real Christianity requires a commitment.
They way I see 1 John 1 is that Christianity is not like a Tesco Clubcard. You a sign up to a supermarket loyalty card and there’s no comeback if you almost always shop elsewhere. Lack of custom won’t cause the security guard at Tesco to turn you away.
The problem with nominal Christianity is that is, perhaps unwittingly, based on the misnomer that it’s possible to be a Christian with little or no life-changing effects. We have to actually “walk in the light”. That can involve giving up some of the things we enjoy (but, I find, ultimately negative). But, hey, Jesus “cleanses us from all sin”!
John continues in chapter one with:
If we say, “We have no sin,” we are deceiving ourselves, and the truth is not in us. If we confess our sins, he is faithful and righteous to forgive us our sins and to cleanse us from all unrighteousness. If we say, “We have not sinned,” we make him a liar, and his word is not in us.
This is a call to be honest, starting with how we think internally, about our sinned. I don’t know if you’ve experienced this, but sometimes other people clearly do something wrong but are adamant they are in the right. As Christians, we should avoid that sort of approach.
There will be times when we are partly in the wrong in a situation – maybe it’s really someone else’s fault but we responded to it badly. So let’s say it’s 80% someone else’s fault and 20% ours. Friends might tell us just to blame the other person. But as Christians, we are called to repent of the bit we’re to blame for.
We shouldn’t seek to justify our self-worth by denying when we’ve messed up. Instead, we should celebrate the fact that Jesus has cleansed us from all our sins.