Here is the Merriam Webster definition of myth (and I have now officially exceeded the number of times allowed in a single day to quote from a dictionary). I offer it simply, for clarity sake, as an a good example of what I personally believe a myth to be:

Main Entry: myth
Pronunciation: 'mith
Function: noun
Etymology: Greek mythos
Date: 1830
1 a : a usually traditional story of ostensibly historical events that serves to unfold part of the world view of a people or explain a practice, belief, or natural phenomenon b : PARABLE, ALLEGORY
2 a : a popular belief or tradition that has grown up around something or someone; especially : one embodying the ideals and institutions of a society or segment of society b : an unfounded or false notion
3 : a person or thing having only an imaginary or unverifiable existence
4 : the whole body of myths

Now, in contrast to that definition (and purely for argument sake), here is the Merriam Webster online definition of religion:

Main Entry: re·li·gion
Pronunciation: ri-'li-j&n
Function: noun
Etymology: Middle English religioun, from Latin religion-, religio supernatural constraint, sanction, religious practice, perhaps from religare to restrain, tie back -- more at RELY
Date: 13th century
1 a : the state of a religious b (1) : the service and worship of God or the supernatural (2) : commitment or devotion to religious faith or observance
2 : a personal set or institutionalized system of religious attitudes, beliefs, and practices
3 archaic : scrupulous conformity : CONSCIENTIOUSNESS
4 : a cause, principle, or system of beliefs held to with ardor and faith
- re·li·gion·less adjective