Christian Post:

“…According to the IRS, charities fall under 501(c)(3) tax exemption status, which means such nonprofit organizations, including churches, synagogues, temples, mosques, etc., are not required to pay federal taxes and can receive tax deductible contributions. For churches, much of their contributions come in the form of offerings and tithes. A tithe is generally considered 10 percent of one’s income. Churches, classified as public charities by the IRS, need not explicitly apply for 501(c)(3) status to reap its benefits.”
http://www.christianpost.com/news/should-tithing-count-as-charitable-giving-74990/#8yI8rrucGYbFAlWL.99