Is the word "deism" capitalized?

Correct!

It is a type of thought or belief but not a religion!

DEISM - Belief in God for natural reasons not from reading the gospel. A belief in the existence of god based solely on natural reason without reference to revelation.

Ignore the BAD Americans that try to claim it is a religion they are just abusing atheists rights under the constitution and human rights!

In the United States, atheism is protected under the First Amendment's Free Exercise Clause. In August 2005, in a case where a prison inmate was blocked by prison officials from creating an inmate group to study and discuss atheism, the court ruled this violated the inmate's rights under the First Amendment's Free Exercise Clause. The United States Court of Appeals for the Seventh Circuit affirmed previous Supreme Court precedent by ruling atheism be afforded equal protection with religions under the 1st amendment.

Clearly setting out that atheism is NOT a religion but IS afforded the same protection as a religion under the constitution!

Its up to you. I generally capitalize religions as a sign of respect.

It's not a proper noun so no.

who cares?

Any ism is capitalized. Whether it is religious or not.