The separation of state and church should matter to you.
It matters to the health of our democracy. It matters to the promise that everyone regardless of religious belief or nonbelief will be treated equally under the law. No religion is above any of the others or can be established by government. You are free to live according to your own dictates. Your freedom of conscience is a protected right.
The wall of separation between church and state, a phrase coined by Thomas Jefferson in a letter to the Danbury Baptists in 1802, is an uniquely American ideal. That ideal, born out of the Enlightenment, was the first of its kind. The United States was the first among nations to protect a person’s right to think freely. Our founders sought to secure individual religious liberty by separating church from state, adopting that principle into our entirely secular, godless Constitution.
This principle, whose boundaries have been safely settled since the adoption of the Constitution and Bill of Rights, is now under attack. When the separation of religion and government is threatened, so are all of the other civil rights issues you care about: the right to abortion, the right to have laws and policies affecting climate change be based on reason and science instead of religious ideology, the right to make life-ending decisions based on your own moral conscience, the right to make your own medical decisions, the right to be free from discrimination based on who you are, the right to a free and secular education, and the right to a secular democracy with equal protection under the law for everyone.
Every single issue you care about is impacted by the separation of state and church. And vice versa.