1. Neither the word nor the concept is innately Christian.
2. Muslims have for centuries had separate secular and ecclesiastical authorities. The majority of Arab/Islamic nations have such separations: The Presidents of Egypt and Syria are not clerics, neither is King Abdullah bin Abdul Aziz Al Saud of Saudi Arabia, neither is King Abdullah II bin al-Hussein of Jordan.Etymology:
Middle English, from Anglo-French seculer, from Late Latin saecularis, from saeculum the present world, from Latin, generation, age, century, world; akin to Welsh hoedl lifetime
1 a: of or relating to the worldly or temporal <secular concerns> b: not overtly or specifically religious <secular music> c: not ecclesiastical or clerical <secular courts> <secular landowners>2: not bound by monastic vows or rules ; specifically : of, relating to, or forming clergy not belonging to a religious order or congregation <a secular priest>3 a: occurring once in an age or a century b: existing or continuing through ages or centuries c: of or relating to a long term of indefinite duration <secular inflation>
3. Atheists themselves are generally (sometimes rudely) aggressive in their championing of secular authority.
4. Christian teachers from St. Paul on down have instructed the Christian faithful to sustain their faith throughout their daily lives; my understanding is that Jewish rabbi's offer similar instruction to their faithful; Buddhists are guided in practicing "simple mindfulness". Each after their own fashion, for every religion, "domination of their beliefs in each and every aspect of human activities is the goal of their activity."
I need read no more than the Sermon on the Mount to marvel at the impiety of your intolerance and narrowmindness. (That whole bit about "blessed are the meek....")