If you look up
religion on Dictionary.com you will get quite a collection of different definitions for "religion". What is most notable is how few of them actually make any mention whatsoever about belief in a deity.
What they tend to have in common is a set of beliefs and practices agreed upon by a collective group of people, and typically some sort of significant devotion to said beliefs and practices. Often some sort of moral code by which members live is included.
Thus religion would appear to be quite possible absent a deity, and belief in a deity would appear to be quite possible absent religion.