Deacon Mark – the priest at the St. Michael's Parish – has been cloaked in an enigma and, thus far, fits the bill to have had a hand in the murder. He was the last person to have talked to Erin on her cellphone, and the circumstances of his transference from his last church raise a red flag as well. He was accused (and later cleared) of being involved in inappropriate sexual misconduct with a 14-year old girl. At the end of episode three, we also see him dump Erin's bicycle in a secluded lake in the middle of the night, which police have been looking out for since the beginning of the investigation.
However, as often is the case, the lead suspect we are supposed to believe to be the culprit does not necessarily turn out to be the actual perpetrator. So, it is my guess that Deacon has either aided the real criminal or he's been sitting on the crucial piece of the puzzle box of this show.