Practically, people form groups by caste, religion, race, region,nation, etc for the benefits of the strength of unity; and also to dominate and exploit other people and groups. People in a group support the activities of the members of the group, even if they have bad effects on other groups.
If two different severely conflicting groups are together, only pseudo-harmony exists between them. No one from an opposite group wishes your well being and no one from
the opposition group feels sad at your fatality. One of the solutions to this issue is reducing the opposition by conversion to your side.
Only one ideology is correct at a time and place. All other ideologies are either to cheat people or due to ignorance. There are many political, social and religious ideologies.
If a person is sticking to a group or ideology or religion, it can be because he gets some benefit out of it lawfully or unlawfully. Many people are not honest. They just think that they are correct. People switch groups when current group is not benefitting and another group is beneficial.
For many people, a religion is also a groupism other than a spiritual path. For many people, spirituality and principles in a religion are only a self excuse to enjoy the benefits of the religious groupism. Some people try to convert followers of other religions in to the self followed religion in order to get convinced that the self followed religion is greater than other religions. Some people convert followers of other religions to reduce enemies. Some people make conversions with love towards others, in order to bring them to better (as per their understanding) path. People may not convert others when benefits of groupism and exploitation of other religious groups are more than the benefit of
converting others (including the other groups in to self group may make the self group poor and there will be no way to exploit the other groups. This is similar to what we see in the groups of dogs). Some governments and political parties encourage or discourage religious conversions for political benefits.
There do not exist two principles being correct and contradictory simultaneously. So, if a religious principle is contradictory to some other religion’s principle, only one of them may be correct or none of them may be correct.