If you are looking to blame someone for the decline of Christianity you could start with the large segment of the population that calls itself Christian yet never steps foot in a church and has no idea what it believes at least until a beer commercial or self help book tells them. Or the wackos with big hair on TBN.
As far as Muslim evangelism in Europe, Christianity in Europe has been waning for decades, the Muslims may as well use the churches, the descendants of Christians are not. Humanism and reason, which in my view can be religions in and of themselves, have replaced Christianity in Europe. The Muslims are evangelizing rootless post moderns not Christians.
I honestly believe that in the free market place of ideas Christianity has nothing to fear from other religions, I do not fear another religion so much as someone telling me I cannot practice my religion. Our current (outside) threat is not from Islam,
it is in fact from our own government trying to limit what can and cannot be said from our pulpits.
Even this, from a historical perspective, is not bad news for a Christian, persecution and adversity have historically
always strengthened the Church. We
should be absolutely fearless.