Many people in Rassilon choose to select a Patron God appropriate to their craft or profession. While not mandatory, having a patron means your god will speak on your behalf when being judged by Scaetha, who will then judge the soul by how well they served the ideals of their Patron.
Selecting a patron does not require that one only worship that god. Rather, any gods appropriate to the moment and the circumstances may be worshiped. The patron is just given the highest precedence by the worshiper, is paid the most honor, receives the most sacrifices, etc and it is by their service to the duties of their patron that they will be judged.
The exception to this is if the worshiper pays homage to a deity who is a mortal enemy of their patron. For example, a worshiper with Thunor as his Patron should never honor Thrym, as Thrym is the mortal enemy of Thunor. Paying Thrym any honor would be a minor or major sin against Thunor.
Individuals may change patron gods throughout their lives if they wish, and the gods bear them no ill will for doing so. It is accepted among the gods that mortals have free will, and are therefore entitled to exercise it. Few deities would therefore bother to speak out against a lapsed follower as a result.