Deeley's decision comes after the council altered its rules for issuing public statements. In February, members voted to base policy positions on a majority vote instead of the unanimous backing of its eight members.
"As the Bishop of the Diocese I find this unfortunate, but I see no alternative. Our continuing participation could result in me advocating for two different, and even contradictory, positions," the bishop wrote in an April 23 open letter published by the Portland Press Herald.
"I do not take this decision lightly or happily," Deeley wrote. "Our corporate religious witness in a secular society has been a value to the people of Maine."
The bishop's announcement proved timely. Not long after the council's rules went into effect, the group announced its support for a state bill to ban reparative therapy, a bill authored by lawmaker Ryan Fecteau, the Catholic University of America's first openly gay student general assembly president.
The diocese of Portland spoke out against the measure, saying it "limits choice" for the same-sex attracted.
But the Maine Council of Churches endorsed the bill, blasting reparative therapy as "psychological and spiritual malpractice" that "amounts to torture."
"Sexual orientation and gender identity are a gift from God — not a condition that needs treatment, not a choice that needs conversion, not something broken that needs repair, not a sin that needs forgiveness," the council said in an open letter to the Portland Press Herald.
Jane Field, the council's executive director, acknowledged a growing divide between the Catholic position on homosexual acts and that of the council's mainline Protestant members, which have been drifting steadily leftward for some time.
"Remaining silent on issues, especially related to LGBTQ justice and equality, wasn't tenable for all of our other seven denominations," she said. "It was enough of a discomfort that it needed to be addressed openly. It wasn't healthy to be silent anymore."
In his April 23 open letter, Bp. Deeley reiterated his responsibility as a shepherd. "What I advocate for cannot be simply determined by a majority vote. It is expected that my advocacy is grounded in the teachings of the Church."
"Any other position would be contrary to my responsibility as the bishop of Portland," he added.
Field expressed regret over the diocese's departure but said she hopes the council can work with the diocese on other issues.
"There's a deep sadness, but at the same time, I feel the council still has a vital role to play in the state," she said. "I believe we will find ourselves side by side with the diocese on certain issues like hunger and human trafficking."
In communication with the council, Bp. Deeley agreed.
"As we do with the many activities of our parish communities," he said, "we will be working to serve the needs of the poor, the disadvantaged and the migrants among us, and keep before the people of our state the need to serve the common good through our care for one another."
The diocese of Portland officially exits the council on June 30.