They have rejected (ēthetēsan). First aorist passive of atheteō, late verb (first in lxx and Polybius), to reject, set aside (from athetos). See note on 1Th 4:8; Gal 2:21.
Their first faith (tēn prōtēn pistin). “Their first pledge” (promise, contract) to Christ. It is like breaking the marriage contract. Evidently one of the pledges on joining the order of widows was not to marry. Parry suggests a kind of ordination as with deacons and bishops (technical use of krima and pistis).