A Christian is obliged to assert that any belief which contradicts their own is a false one. However, the scriptures also give indication that many may be sanctified by their actions and that they should judge not lest they be judged. So if, to a Christian, Jesus is THE way, THE truth and THE LIFE, then anyone who does not accept Jesus IS wrong. Does this give a Christian the right to be moralistic, judgmental or condescending to others? No, because all have fallen short of the glory of God and therefore none are beyond reproach. If, however, Jesus is the way, the truth and the life to them then all who sincerely follow Him are following the right path which leads to salvation and their lives are supposed to be exemplary. Thus, in an ideal situation, a Christian community would be living the way which God has intended and all who do not live this way ARE therefore in the wrong. However, we know that sin exists and that not all will accept the message and live 'the life'. In this respect the scriptures say that these others should just be left to their own devices. So Christianity, to a Christian, is not a social norm but more of a presently unattainable > ideal < social norm, a form of 'utopianism'.
The last part of your question is phrased poorly and doesn't read as it was possibly intended.
Did you mean:- "that Christian beliefs are the social norm to the preclusion (or exclusion) of outsiders' cultural heritage and religious practices." ??
If this is what you meant then the answer is that it is an internalized thing and Christians should just avoid engaging in sinful, immoral or inappropriate practices or beliefs rather than taking active steps to suppress or abolish them unless they represent a direct threat to Christianity itself or a direct threat to humanity, as a whole, or if they represent a serious violation of fundamental freedoms or rights.
So whereas the scriptural argument is to maintain justice and do what is right (according to God) this has to be weighed very carefully against infringement upon fundamental freedoms and rights of individuals. This does not prevent Christians from practicing these commands within their own ranks and in many ways there is a much stronger case as to why they should.
Are Christians "Above the Law"? Yes, they are. They are under the grace of God and the grace of God will always supercede the Law. However, we know that if Christians break the Law they may face punishment for upholding their beliefs. The net result being that whereas they are PUNISHED by the Law, they will be REWARDED by God for, as it says, quite rightly, Blessed are they who are persecuted because of righteousness.
So if Christians have to make the choice between obeying God and obeying man (or the world) .....
There is no choice to be made.
Having said that, there are occasions where it may be necessary to make concessions, in the name of common sense and in order to demonstrate compassion and humanity to the glory of God. (letting one's gentleness be evident to all)**. But the warnings are there that too many concessions lead to rebellion and rebellion against God can only lead to one thing and that is death. So even if these concessions are made, it should be pointed out, with absolute clarity and in accordance with righteousness, what the right path is. After all, is it not kinder to speak the truth rather than let someone die of ignorance?
** NB This, quite obviously, means that any acts of violence or aggression, unless they are FULLY justified, namely in self defense, in defense of sanctity/righteousness (limited aggression), or in defense of the innocent, are not allowed. (A time for all things under the Heavens - Ecclesiastes 3)