A priest changed one word. Thousands of baptisms he performed are declared invalid.

A priest resigned this month after his diocese announced that thousands of baptisms he had performed were invalid because he had changed a single word. He said, "We baptize you ... ," instead of "I baptize you ... ."

"It is with sincere pastoral concern that I inform the faithful that baptisms performed by Reverend Andres Arango, a priest of the Diocese of Phoenix, are invalid," Bishop Thomas J. Olmsted of the Diocese of Phoenix said in a letter last month.

"This determination was made after careful study by diocesan officials and through consultation with the Congregation for the Doctrine of the Faith in Rome," he wrote.

Arango said, “We baptize you in the name of the Father, and of the Son, and of the Holy Spirit," instead of "I baptize you in the name of the Father, and of the Son, and of the Holy Spirit."

"The issue with using 'We' is that it is not the community that baptizes a person, rather, it is Christ, and Him alone, who presides at all of the sacraments, and so it is Christ Jesus who baptizes," Olmsted said.

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The Vatican’s Congregation for the Doctrine of the Faith in 2020 clarified that using "we" during the sacrament made it invalid.

Arango has been a priest and performing baptisms since 1995, according to Katie Burke, a spokeswoman with the Diocese of Phoenix. He also practiced in San Diego and Brazil.

"It is my understanding that Father Arango was using the incorrect words from the beginning of his priesthood (1995) until it was brought to the attention of the diocese last summer," Burke told NBC News in an email. 

She said she didn't know precisely how many people were affected but believes the number to be in the thousands.

Baptisms performed by Arango after June 17, 2021, are presumed to be valid, Burke added.

In a letter to his parish, Arango wrote: "It saddens me to learn that I have performed invalid baptisms throughout my ministry as a priest by regularly using an incorrect formula. I deeply regret my error and how this has affected numerous people in your parish and elsewhere."

He said he resigned, effective Feb. 1, so he could "dedicate my energy and full time ministry to help remedy this and heal those affected."

"I sincerely apologize for any inconvenience my actions have caused and genuinely ask for your prayers, forgiveness, and understanding," he wrote.

The diocese said Arango "has not disqualified himself from his vocation and ministry" and "remains a priest in good standing."

Those who believe they or their children were baptized by Arango can fill out a form online to be properly baptized. Subsequent sacraments, including marriage, may need to be repeated by those who had invalid baptisms performed by Arango, according to the Diocese of Phoenix.