That is an excellent question. Christianity in the West certainly is in a dilapidated state, you are correct! I'm not completely sure of the answer, it's multifaceted. However, I would say this, I believe the main reason is simple...the church stopped believing the Bible as the Word of God. That was the foundation that began to erode the Christian church in the west and then spread into western society at large. Now we are LOST!!! We don't have a unifying 'Truth' any more.
Of course, Islam has a VERY strong central pillar that the Koran is the very words of Allah, spoken to the Prophet. Am I right? That is an exceedingly strong centralising core that helps Islam remain strong.
However, to say that Christianity is 'completely' dilapidated is probably an overstatement. Christianity in many places is resurgent. The church in China, India, Africa, and South America is exploding and growing rapidly. Even in the Middle East and former Russian states throughout Asia Christianity seems to be growing again. Christianity seems to grow when it is persecuted and marginalised. The more Christians suffer for Christ, the more they hold fast to the faith and spread it.
Islam had a golden age from its inception in 6th Century to the 11th Century. It was incredible how fast and thorough it spread throughout the Middle East, Asia, and North Africa, and Eastern Europe (Which were largely Christian beforehand, pagan before that). Then it retreated for many years. The Ottomans seemed to be the exception of course. You may be able to enlighten me on other Islamic revivals throughout history as I'm not familiar with Islamic history as much as Christian.
But you are right. In the West, Christianity is in a dilapidated state. I contend that it is because Christians in the West are comfortable and lazy in their reading of the Bible and most importantly, believing it!!!
