PORTLAND, Maine — The Roman Catholic Diocese of Portland will not actively campaign against a statewide referendum seeking to legalize same-sex marriage but instead will focus on teaching parishioners about the sanctity of marriage between a man and a woman, the bishop announced Friday.

Bishop Richard Malone unveiled a 22-page pastoral letter titled “Marriage: yesterday…today…always” at a press conference at the Chancery in Portland. Malone said he wrote it to explain the church’s position on marriage. The document will be discussed at Catholic churches and schools and through the diocesan magazine and radio station.

“What they are doing is appropriate,” David Farmer, spokesman for the Freedom to Marry Coalition, which supports the legalization of same-sex marriage, said in response to the bishop’s announcement. “That’s what they should do.”

Malone said the letter will be the heart of the church’s response to gay marriage supporters. It is the third pastoral letter the bishop has issued since his installation in 2004, according to Sue Bernard, spokeswoman for the diocese.

“I was inspired to write about the beauty of marriage because many people have forgotten the unique and particular qualities that must be present to constitute a marriage, including having the intention of raising a family,” he said in a press release issued after the press conference. “It is also timely because once again, marriage has been thrust into the political arena as a result of the upcoming referendum.”

More than two years ago, Maine voters rejected gay marriage in a statewide vote, 53 percent to 47 percent, after it had been passed by the Legislature and signed by then-Gov. John Baldacci. During the 2009 successful referendum to repeal same-sex marriage, the church gave $500,000 and lent its public policy director full time to the campaign.

Last month, the Secretary of State’s Office confirmed that enough signatures have been verified to allow voters in November to decide whether same-sex couples should be allowed to marry in Maine.

“We’re going to be involved in the issue and vigorously, but in a much different way than we were the last time around,” Malone said in an interview last month in Bangor. “My main thrust is going to be to take my role as the chief teacher of the diocese seriously and to try more effectively to teach and inform Catholic people about our teaching on the nature of marriage and why marriage is the union of one man and one woman — not as an attack on gay people but to say what marriage is, both from what we know from natural law and God’s revelation in Scripture.”

Malone said Friday that the diocese would not be joining the Protect Marriage Maine political action committee being formed to raise funds to oppose the same-sex marriage referendum. The diocese was part of the PAC formed to repeal the same-sex marriage law.

The Christian Civic League of Maine will be a part of the Protect Marriage Maine PAC, which is expected to file paperwork next week with the Secretary of State’s Office, Carroll Conley Jr., executive director of the league, said Friday.

“We have the utmost respect for Bishop Malone and welcome [the diocese’s] contribution,” he said. “We agree that an educational mission is a long-range goal and not necessarily tied to a political cycle, but [the bishop’s commitment] to marriage as one man and one woman will benefit us.”

Conley said that although he and others who oppose the referendum would have liked to have had the bishop and the Catholic Church involved more formally in the upcoming campaign, “we stand with him and the 185,000 Catholics in Maine on this issue.”

The Associated Press contributed to this report.

To read Bishop Richard Malone’s pastoral letter, visit BeautyofMarriage.org.