Is it immoral to tell a person that her religion is wrong?

If religion is about a way of life, then certainly it would have so much of influence on the way a person lives. A religion that has nothing to do with our lives is a useless religion. The way we eat, dress or conduct our moral behaviour are all influenced by our religious preference. If we observe people of different religion and compare their way of life, it is not difficult to spot such differences. Yet we all do have common practices too. 

A Muslim would not eat certain food item. And that is true for a Hindu too. Jains are vegetarian in complete contrast to Christians who have no religious sanction for avoiding any food items except for health reason. Sikhs wear turban as part of their religious tradition; Muslims women do wear burqa and one often comes across Muslim men wearing topi (cap) as if to express their differentness. Christians and Hindus do not have any particular dress code that is binding on them as it is with the Muslims and Sikhs. 

Christians are to work for six days, and on the seventh day to rest and gather for corporate worship. This concept of observing Sabbath Day is a very important feature of Christian practice. Buddhist and Hindus do not have such a day in a week as in the sense of Christians' Sabbath Day. Muslims gather together for prayer on Friday till noon, and the rest of the day can be as other days so that you can even conduct business and other works.Muslims are allowed to have upto four wives provided certain conditions are fulfilled. But such polygamy is prohibited for all the other religious systems. 

But all these differences in practice emerge, so to speak, from differences in doctrinal or metaphysical belief. Christians believe in Jesus Christ, the incarnated God, as Lord. And the language of the Bible too incarnated in the world in which it was born. Therefore, there is no sacred language in Christian belief. Since incarnation is a key concept, Christians can 'incarnate' into any culture. Thus, there is no particular dress code or food habit etc for Christians. Except in moral areas Christianity 'incarnates'  into any culture. For Muslims, Allah cannot take on human form, and  Arabic is the language for Quran. This Arabic cannot incarnate into other culture. Thus we find elements of Arabic culture and language  wherever Islam spreads. Hindus belief about deity has been so diverse, that there can be millions of belief or no belief in a deity. Atheism, theism, pantheism, panentheism, animism or deism can all be appropriated into Hinduism.

Buddhists' nirvana is cessation of suffering. Hindus' moksha is getting out of the karmic cycle. Jains and Sikhs have similar understanding of liberation. Christians and Muslims' salvation is about being in God's presence forevermore. Buddhism is not world affirming as much as Christians, Muslims and Sikhs are. Hindus and Jains have different patterns for different people. And thus these religions engagement with the world or lack thereof differ. And none of the religions have such thing as Sharia Law as is found in Islam.

So we have even the purpose or goal of the religion itself different from one another. And therefore we have such diverse ways to reach the goal. Are all the goals correct? No way. Unless we live in madhouse no one with sane mind can say all the goals are correct. Now one can say, "you pursue your goal and follow your ways to reach your goal." But suppose there is no God as Jains or Buddhist would say, the very goal of Christians and Muslims are incorrect. So is it then the obligation of Jains and Buddhists to show others the right way? But if indeed your belief -- goal and ways, are true, and you remain silent about it,not telling others the right one, you are being immoral. Well, like other aspect of life, one must be respectful in telling that she is mistaken about her way of life (religion). But religious conversion or helping people towards the correct path is a moral obligation. To insist that all religious goals and ways are equally correct is just to render all religious belief and practice meaningless.

If there is no scope for conversion or reformation or refinement, then religious dialogue is superflous. If we are to cultivate religious harmony, inter-religious harmony is important. And when such inter-religious dialogue takes place, introspection results and the next step is refinement of one's religious traditon. Or sometimes one has to make drastic reformation and sometimes convert.