What is its core idea, in one line?

There is no god but Allah, the One (Qur'an 112:1–4), and Muhammad ﷺ is His messenger (Qur'an 48:29) — so worship the One who made you, alone, in the way He revealed.

Atheists generally hold that available evidence is not enough to justify belief in a deity.

Agnostics generally say humans do not have enough certainty to prove either that God exists or does not exist.

The New Testament presents Jesus as the Word made flesh, crucified, and raised from the dead.

The Torah teaches that Israel is called to live in covenant with the one God and follow His commandments.

The Buddha’s first teaching presents four noble truths: suffering, its cause, its ending, and the path. (Dhammacakkappavattana Sutta)

Many Hindu traditions teach liberation (moksha) from rebirth through dharma, karma, devotion, knowledge, or disciplined practice.

The Sikh scripture opens with Ik Onkar: there is one God. It also emphasizes truthful living and remembrance of God.

Secularism generally holds that government should treat citizens fairly regardless of religion and should not enforce one religious doctrine.

Humanists generally believe people can build good lives and better societies through reason, empathy, and cooperation.

Liberals generally believe individuals have rights and that government should protect those rights with consent and law.

Feminists generally believe societies have often treated women unfairly and that laws, culture, and institutions should correct this.

Nationalists generally believe each nation should protect its identity and, in many cases, govern itself.