The muslims believe the christians are idol worshippers among other things. They see the crucifix, the image of Christ on the cross, and believe that this is God. I understand their complaints.
The difficulty with religion is that it is an extremely abstract concept and these high concept abstractions are often lost on those who cannot follow it. They're best just told what to do to live a good, cooperative life with their fellow man as it's likely beyond their ability, intellectually and spiritually to truly grasp the concept of God.
The taoist acknowledge this. In fact, their sacred book the Tao Te Ching, begins with a warning. The Tao which can be told is not the eternal Tao. As nebulous and open to interpretation as the tao is one thing it is not is a noun.
God can only be idolized by first making Him a noun. A noun can be pictured in the mind and a person prays to christ on the cross, or to the image of christ on the cross as has be indelibly burned into their psyche. But god is ephemeral, best understood as a non-thing, both subject and verb and there lies the impossibility for language to grasp God and the impossibility of materialist and scientists to quantify God. If they struggle to understand quantum physics or string theory while denying anything immaterial then they'll never be able to understand God.
Nevertheless, in the mind's of the believe God often becomes an image. Where as religion is intended to bring union with God, a shift in consciousness, it is often lost on those who exploit its precepts or can't understand it.
“You shall not make for yourself a carved image, or any likeness of anything that is in heaven above.... Exodus 20:3
Idolatry worship is in the mind. This is where the muslims get it wrong. The image of the cross is not needed. Ideally, in prayer a Christian would never hold a picture of jesus in his mind nor an image of Michelangelo's God the bearded father in their head. The buddhist get this right. They hold no image of Buddha. That is idolatry as it limits God to a noun.