Christianity isn't the issue, with the hundreds or thousands in specific countries. Atheism/agnostism is the way bigger threat.
Indeed, christianity is already dead.
The religion of liberalism which is atheism is the bigger threat. So why do you constantly lie about secret underground thousands strong christian communities, lol ridiculous lie.
As for atheism, yes since christianity has collapsed liberalism is the only other real challenger to Islam.
Although liberalism itself has run its course, it is already on a decline and Muslims through dawah have successfully undermined its underpinnings.
Due to their efforts in dawah they have become some of the most popular speakers online debating and challenging proponents of liberalism/atheist/democracy etc. Youth in the west including non-Muslims have grown up listening to them, and for those that did not revert to Islam the others have tried to adopt Islamic positions and arguments against the current world order while claiming it should be christianity that replaces it, they are trying and failing to resurrect christianity.
So alhamdulilah, thanks to Allah SWT first and the work of da'ees liberlism/atheism has fallen out of favour and now everyone is trying to adopt religious language.
Islam is at the centre of every discussion now and at the core social change especially in the west.
Islam wins again, Islam always wins.