If it's truly secular, then it won't matter who's in the numeric majority. I have my doubts about the long-term sustainability of such an arrangement in a modern Muslim-majority nation, however.
Islam has never, ever created nor sustained anything that is even slightly compatible with the modern definition of a secular state, and has generally only been dragged to a place of equitable power-sharing with non-Muslims when it has to, as at Ta'if, Saudi Arabia in 1989 in the negotiations to end the Lebanese Civil War.
The fundamentals of Islam in all of its traditional sources on such matters such as the Constitution of Medina (supposedly authored by Muhammad himself) or the Pact of 'Umar are pretty much the exact opposite of what they would need to be to create such a society, so it's not like this is a mistake or that Muslims just haven't been allowed to develop properly to reach the point of having true secularism. Secularism is antithetical to the traditional Islamic view of the world.
To be frank about it, the more Muslims you generally get in a given society, the more the western idea of a secular society goes out the window, as we are seeing happen now in more and more places. What the offer instead are at best essentially modifications on the Turkish millet system, which I'm pretty sure don't satisfy anyone's idea of what a truly secular and pluralistic society ought to look like.
