If you agree with a theory or belief, you subscribe to it, just as you subscribe to a magazine.
Ascribe is a very different word. If you ascribe a belief to someone, you are attributing the belief to that person, perhaps wrongly.
Read about the new edition of Common Errors in English Usage on the Common Errors blog.
If you are not ready for a change, you can still order the second edition at the discounted price of $12 (while supplies last).