What is secularism? It is a principle of state power. In short, religion should be separate from the state. The state and its institutions must be independent. The processes of the state should not be affected by faith and conversely. Thus, the two must not stand in the way of each other.
However, it does not mean atheism. Civil laws are in practice rather than religious laws. The system protects both believers and atheists. Hence, it ensures freedom of religion to all. Thus, we can not count restrictive regimes as secular.
So, it is one of the vital features of Western democracy. Thanks to this purpose, every citizen has the same rights. Nobody can benefit or be persecuted due to their beliefs. We also distinguish between “soft” and “hard” forms. While the “hard” form requires complete separation. For example, no religious laws or proposals are allowed, the “soft” form is more tolerant.

So what is the history behind this idea? A fight for religious freedom began in the Middle ages. However, only the late 18th and 19th centuries mark the true beginning. The author of the term is British man George Holyoake. The USA and France were the pioneers! Since then, it became a part of the modern world.
Secularism in practice
It is most typical for western countries, for example, the United States and Europe. Some states have it in their constitution. A total of 96 are officially secular. Some countries claim this principle but have non-secular tendencies. A politician well-known for his secular approach was Kemal Atatürk. It was one of his most important goals for free Turkey.
Now about the opposite. In a non-secular state, one belief is spinal. That was common in the Middle Ages. Further, there are currently about 51 of these. Often, those countries base their laws on a spiritual script. For example, Islamic countries use Sharia law. It is purely Islamic law based on their tradition. Yet, other faiths face hatred in such states. However, it is not a rule.