A.R.S. § 12-2201: Persons who may be witnesses

A. Every person, including a party, may testify in any civil or criminal proceeding, or before any person who has authority to receive evidence, except as otherwise expressly provided by law.

B. A person shall not be incompetent to testify because he is a party to an action or proceeding or interested in the issue tried, or because he has been indicted, accused or convicted of a crime, or because of his religious opinions, or because he does not have any religious belief.