What does Islam say about secularism?

Two kinds of secularism

The word gets used in two very different ways, and the Islamic answer is different for each:

• Secularism as a worldview: the idea that God and religion are irrelevant to how people should live, and that faith is just private sentiment. Islam disagrees with this. For a Muslim, prayer, honesty, money, family and work all belong to worship.
• Secularism as a political arrangement: the state doesn't adopt an official religion and treats all citizens equally regardless of belief. This is how most of Europe and the USA work, and it's what lets mosques open, hijabis work and converts change religion freely.

Islam is a whole way of life

The Quran speaks about worship, but also about contracts, inheritance, justice, trade and how to treat neighbours. So a Muslim doesn't switch off their faith at the office door. That's not about forcing anything on others. The Quran itself says faith can't be forced:

Living by Islam yourself and respecting other people's choices go together. See Is there freedom of religion in Islam?

Living as a Muslim in a secular country

Most scholars living in the West agree that Muslims can be loyal, law-abiding citizens of a secular state. You keep your agreements, pay your taxes and follow the law, unless a law forces you to commit a sin, which is rare in practice. The early Muslims who fled to Abyssinia lived safely under a just Christian king, and the Prophet ﷺ praised that land for its justice. See Do I have to follow the laws of my country as a Muslim?

What scholars debate

Scholars differ on what the ideal Islamic state looks like and how sharia relates to modern constitutions. Some see secular democracy as a practical, acceptable framework where Muslims can still promote good; others hold that a Muslim-majority state should be guided by Islamic law. Almost all agree that Muslims in non-Muslim countries should work within the system peacefully. See Is democracy compatible with Islam?