It is not yet clear whether Mattson will face charges in connection to the man's allegation.
Mattson's book is published by Ignatius Press.
In a Jan. 30 statement, Ignatius said it "was unaware of this alleged misconduct until the accusation was made this week."
"Dan has been forthright about the sinfulness of his past life and he has sought to encourage people to find new life in living chastely, according to God's purpose for men and women. In publishing his book, 'Why I Don't Call Myself Gay: How I Reclaimed My Sexual Identity and Found Peace,' we understood from Dan Mattson's own account of his life that before he became a Catholic he engaged in many sinful sexual activities, which he has repented of. We recognize that what is alleged to have occurred happened before Dan Mattson's conversion to the Catholic faith. Yet, this allegation concerns a kind of sinful activity that also has civil law implications and it involves harm to someone who was a minor when it allegedly occurred, which is deeply troubling."
"We at Ignatius Press are praying for all concerned, especially the alleged victim, and we desire a healing and a just resolution to the matter. At the same time, we recognize Dan Mattson as a man who has publicly repented his past sins and is actively engaged in helping others to avoid a way of life that he fully recognizes is sinful and profoundly harmful."
Father Philip Bochanski, executive director of Courage, said he was "devastated" to learn of the allegation against Mattson.
"The same afternoon that I learned of this very disturbing situation, I immediately reported the information I had to the Safe Environment Coordinator of the Diocese of Bridgeport (where the Courage Office is located) and to the Child Protective Services office of the State of Michigan (where Dan resides), as I am required to do by civil law and diocesan policy," he said in a Jan. 30 statement.
Mattson has never been an employee of Courage International, but has shared his story at conferences organized by Courage, and in "Desire of the Everlasting Hills," which Courage International produced, Bochanski said.
He noted that the alleged misconduct occurred "more than 14 years ago, years before [Mattson's] involvement with the Courage apostolate," and said that neither he nor his predecessor, Fr. Paul Check, had previously received sexual allegations against Mattson.
"Mattson will not be invited to speak or write, or to take any leadership positions, on behalf of Courage International for the foreseeable future. I will reserve further comment on this matter until the civil authorities have made a final determination in the case."
"I know how painful this news will be to many people, particularly those who are survivors of abuse in their own lives or in the life of a loved one," Bochanski said.
(Story continues below)
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"I am praying earnestly for a just resolution to this matter, and particularly for the needs of the man who has brought this situation to light. I urge anyone who is aware of an incident of sexual misconduct or any kind of abuse involving a child, youth or other vulnerable person, to contact civil authorities immediately."
This story is developing, and was updated Jan. 30 at 5:30 pm MT.