What do you think of the idea that christians who stopped believing were never really christians? [Spirituality & Religion]

Nobody IS a Christian, it's an ideological choice, as well as a cultural habit. Both things can be broken, although the memory remains.

It's a convenient excuse used by Christians to explain someone's differing growth and awareness.

Real talk, this is actually a serious issue. Should you the reader be a non Christian, this is not directed as a barb at you and DEFINITELY not an attack on your beliefs. I’m just directing this at OP and fellow believers like me.

Here’s the unhelpful Philosophical answer:

There is an ideologue divide between those believe this who Accept Jesus and are saved for good. And those who think believers can fall from grace. Whether the former idea is true or the latter or somehow both is not really helpful, because then we ask: am I saved or not? I accepted Jesus how long if any before I’m permanently “safe”

The practical answer:
If you are a believer: hold onto Jesus and don’t let go: We are told in Romans 8:38-89 St. Paul says “For I am convinced that neither death nor life, neither angels nor demons, neither the present nor the future, nor any powers, 39 neither height nor depth, nor anything else in all creation, will be able to separate us from the love of God that is in Christ Jesus our Lord.”

The question is not IF God loves you for he loves you so much he sent his Son to redeem you and welcome you home. The only question is: DO YOU love God in return? You can never pound for pound reciprocate that grace or pay back God. But we can still show our love by spreading the good news, keep brothers and sisters and others in the faith and walking in personal obedience with HIM.

By and large you cannot out-sin the cross. Its just up to you to keep your faith and live in the grace you have received.

Peoples religion (if they have one) is very much dependent on geography!!

It's a rationalisation to defend a presupposition. And it isn't justified by what the bible says.

Passed a lagoon in Papua New Guinea with 'beware of crocodile' sign on OPPOSITE side to where tourists came. A big old salt water croc. I laughed. My local colleagues dismissed it. 'Crocodiles aren't dangerous.'
'Ummm... yes they are. Lots of people in Australia get killed by them. Mostly German tourists who ignore the signs.'
'Nah. Crocodiles only kill you if you're cursed.'
That was interesting. 'So... how do you know if you're cursed?'
Without looking up, he said, 'You get eaten.'

You can laugh at their world view, but it works. And is our world view superior? They lived in harmony with crocodiles for millennia. Our world view demolishes entire eco-systems every decade. Maybe our world view needs more scrutiny, and isn't as rational as we'd like to think.

I agree. When we accept Jesus payment for our sin on the cross, His Holy Spirit comes into our heart. He promises to never leave or forsake us, so we cannot go from being a Born Again Believer to a non believer. Those folks never did believe in their hearts. Maybe in their mind, but that is not enough.

It's the truth.