Is Idol-worship allowed in Christianity? If no, why there are idols of Jesus, Mary etc in church? And why people worship their idols.

They are not idols in the sense of being other then what they are. They are representations of the persons they represent. There is no idol worship going on.

There are no idols in any Christian church, but I will assume you really know this anyway.

Also I will assume your real purpose of this question was to stir up trouble on the board.

But some churches, Catholic, Orthodox, Anglican and some others will use statues and/or icons of various saints, and of Jesus to be used in prayer, to be used to remember their lives as an example to us.

There is no idols or idol worship