A heretic

one with controversial opinions about religion, politics, etc. extremely rebellious against government

n 1: a person who holds religious beliefs in conflict with the dogma of the Roman Catholic Church syn: misbeliever, religious outcast
2: a person who holds unorthodox opinions in any field (not merely religion)

Invented with the creation and spread of Christianity, a heretic (in the middle-ages) is every one who did not accept or questioned the Catholic church or its laws. Unlike what the Church spread, Heretics were rarely Satanists or non-believers.
In fact, in many cases, they had a stronger and more realist belief in God than that of the Church followers. It is just that the Heretics believed the Church's ways were wrong and corrupted, and that is why they followed their own way.

Usually applied by Warhammer 40k fans or members of the Adeptus Astartes, Adeptus Custodes, Inquisition, or Exterminatus Extremis to anyone who dislikes the Warhammer 40k universe, and/or acts/says in a negative way towards the Imperium.

Anyone who speaks or acts against the religion of The Covenant.

To the best of my knowledge, this words original meaning is to "be different or to have different beliefs". A common misunderstanding is that anyone who is a heretic is more than likely a satanist and enemy of God. This is not always true. For me, it just means "Different".

being of or partially of a certain nationality when you or a previous family member was once a resident of said country; being partially or entirely fluent in the native language of your nationality which includes having the ability to speak more than a few words here and there