Bishop William Murphy celebrated his final Mass on Sunday as the head of the Diocese of Rockville Centre.

Murphy, 76, has been the diocese’s bishop since 2001 — he began his tenure just days before the World Trade Center terrorist attacks.

The diocese’s new bishop, John Barres, will be installed on Tuesday as leader of Long Island’s 1.5 million Catholics. Barres, 56, will become the diocese’s fifth bishop since its founding in 1957 and will oversee Rockville Centre’s 133 parishes and 57 Catholic schools. He was one of the youngest bishops in the United States when in 2009, at age 48, he was tapped to head the Diocese of Allentown in Pennsylvania.

On Sunday at St. Agnes Cathedral, Murphy barely mentioned his upcoming retirement, though he asked the more than 200 parishioners at the 11 a.m. Mass to pray for Barres. Murphy resigned from the Vatican last year upon his 75th birthday, following Holy See protocol, but remained in his post until Barres was named as his successor on Dec. 9.

“Mass isn’t about me,” Murphy said after Sunday’s service. “Mass is about the Lord and his people. I’m the go-between.”

During his 13-minute homily, Murphy asked the parishioners to accept Barres into their church with “gratitude” and “thanksgiving.”

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“This Tuesday, we are blessed to welcome our new shepherd,” he said.

Murphy began his tenure in Rockville Centre on Sept. 5, 2001. Days later he visited families and donated blood at Mercy Medical Center, though he had to wait for his blood pressure to decrease. As Long Islanders failed to claim their vehicles left in the village commuter parking lot across the street from the church — a sure sign they’d been killed or wounded in the attacks — Murphy hosted a Memorial Mass at the Nassau Coliseum to honor the victims. But he said he was struck by the response of the survivors.

“I have never seen people more generous and kind to each other,” he said.

Murphy’s tenure has not been without controversy. Most recently, he wrote a letter read at Masses days before the 2016 election that any political candidate’s support for abortion should disqualify him or her from the Catholic vote. A diocese spokesman later said Murphy’s letter was nonpartisan and the bishop was not endorsing Donald Trump.

On Sunday, parishioners lined up outside St. Agnes in breezy weather to shake Murphy’s hand and thank him for his service.

“He always had a smile,” said 20-year parishioner Bob Tolan, 52, of Rockville Centre. “He was a very active, hardworking shepherd of our parish. ”