Contamination-miasma-breaks the relationship between Gods and men, and the purification restores it, restoring also the natural course of things. The ceremonies of purification have the general name of ‘katharmoi’, whose task is to restore the religious order, either daily or in specific circumstances; ‘katharos’ means clean, tidy, both in a concrete and in an ethical sense- from which results that any material purification has no meaning if, together, we do not purify ourselves inwardly through the constant practice of the rules of Eusebeia. For example, the couplet written at the entrance of the temple of Asklepios at Epidaurus: “Those who enter the fragrant temple must be pure. To be pure means to have holy thoughts” or “may ritual and moral purity be present at the perirrhanteria and beyond the doors of the temple. “
Since the causes of contamination are many, these are many means and rituals of purification, but they all share on one hand, the cancellation of the state of impurity through specific practices, on the other, the finding of and removing the causes which provoked the impurity.
The purification is a prerequisite for any ritual: the sacrificer must be pure body and soul; in the various Hellenic and Italic shrines the purity standards were engraved on stone at the entrance of the sanctuary itself; especially important for the behavior that must be observed to keep ourselves pure in respect to the Gods, the priests, the sacred places and in every activity of worship were the rules of the ‘Sacred Laws’, which did not undergo any change from the archaic to the imperial period.
Rest of the article, including some interesting details about purification means and rites, here.