Secularism in the modern political meaning – the idea that religion and political authority, church and state are different, and can or should be separated – is, in a profound sense, Christian. It’s origins may be traced in the teachings of Christ , confirmed by the experience of the first Christians; its later development was shaped and, in a sense, imposed by the subsequent history of Christendom. The persecutions endured by the early church made it clear that separation between the two was possible; the persecutions inflicted by later churches persuaded many Christians that such a separation was necessary.
The older religions of mankind were all related to –were in a sense a part of-authority, whether of the tribe, the city, or the king. The cult provided a visible symbol of group identity and loyalty; the faith provided sanction for the ruler and his laws. Something of this pre-Christian function of religion survives, or reappears , in Christiandom, where from time to time priests exercised temporal power, and kings claimed divine right even over the church. But these were aberrations from Christian norms, seen and reciprocally denounced as such by royal and clerical spokesmen. The authoritative Christian text on these matters is the famous passage in Matthew 22:21, in which Christ is quoted as saying, “Render unto Ceasar the things which are Ceasar’s and unto God the things that are God’s”
Commentators have differed as to the precise meaning and intention of this phrase, but for most of Christian history it has been understood as authorizing the separate coexistence of two authorities, the one charged with matter of religion, the other with what we would nowadays call politics.
I do believe this is the balance that the US tries to achieve and what Islam perhaps cannot, to date. Although relatively serious reforms by the Ottomans were attempted prior to WWI.
The aberrations from Christian norms that support separation of Church and State were overcome by Christianity itself. That is one of many reasons why Christianity continues to experience success in our modern (soon to be unmodern) era.