04/13/2009 at 06:00 PM EDT
On Easter Sunday, Mel Gibson attended Mass at the Roman Catholic church he had built in 2005. His wife wasn't with him. And after the service, he quietly spoke to some parishioners.
"Well," he was overheard saying, "she's filed for divorce."
Last Thursday – the day before Good Friday – Robyn Gibson, 53, his wife of 28 years, signed divorce papers
, seeking joint custody over their minor child, Thomas, 9, and spousal support.
That wasn't the only surprising news. According to a response that Gibson's
lawyer filed Monday afternoon, the couple have been officially separated since August 26, 2006, almost a month after Gibson had reached a personal nadir when he was pulled over for a DUI in Malibu.
The movie star regained his sobriety following the notorious roadside
in which he made anti-Semitic remarks that he later apologized for.
Robyn's initial divorce petition lists the date of separation as "to be determined."
"They'll do this amicably," a source tells PEOPLE. "There's a lot of mutual respect and they've worked hard on their marriage and worked hard on their
Adds the source, "They'll handle the divorce in the same respectful way."
In fact, Gibson and his estranged wife attended church together with other family members on Palm Sunday, just four days before Robyn signed the divorce papers, PEOPLE has learned.
In addition to Thomas, the Gibsons have six older children: Milo, 19, Louis, 21, William, 24, twins Edward and Christian, 26, and daughter Hannah, 28.