Though there are many different denominations of Christianity, which differ on the stress that they put on certain doctrines, all of them share common beliefs which are at the core of Christianity. These beliefs include the trinitarian aspect of God, that there is only one God, that Jesus was His Son, and that salvation is only possible through accepting Jesus's sacrifice on the cross and subsequent Resurrection. To deviate from these beliefs is to deny being a Christian.
Beliefs, however, should result in actions which reflect them. This is where the potential for error lies. While it is true that many other religions advocate principles that are shared by Christianity; principles such as charity, love, and humility, among others, and should therefore be respected for their contribution, their motivations for these positive actions stem from different beliefs, which cannot be accepted by Christians as being true or valid if they contradict the basic tenets of Christianity. We tend to confuse the actions exhibited by adherents of different religions as being the core of the religion itself, rather than the beliefs which inspire those actions. It is a simple mistake for people to make as the external actions are more easily noted than internal motivations. However, we should do our best to correct our deficiency and try to analyze a person's inspiration for their actions.
This is not to say that Christians are exempt from respecting people whose beliefs differ from theirs. If people refused to accept people whose opinions differ from their own, society would not be able to function. Respecting people and their beliefs is not the same as accepting their beliefs as one's own.
Unfortunately, it has become common for American society at large to stress the similarity between religions in an effort to establish harmony among the different religions. Though this is a laudable goal in itself, it has also caused much confusion among people, who have taken this to mean that all religions essentially believe the same thing. Since it is their differences that define them, the various religions, if they wish to maintain their individuality, must hold on to what sets them apart to survive. In doing this, they must also be careful not to disparage other religions, a natural tendency when reacting against such a widespread belief, but one that is anathema to both those outside and inside Christianity.