DOWNTOWN BROOKLYN — A federal judge on Wednesday declared a homeless man — who allegedly desecrated a crucifix at St. Athanasius Church in Bensonhurst and is accused of trying to light fire to a Brooklyn synagogue — mentally unfit to stand trial.

The defendant, Ali Alaheri, 30, was ordered to spend 90 days in a psychiatric treatment facility before he’s evaluated again for competency, Brooklyn Federal Judge Rachel Kovner ruled.

If he doesn’t take his medicine, the court will review whether he needs “involuntary treatment,” the judge decided.

Alaheri is charged with a hate crime for busting up a statue of Jesus affixed to a wooden crucifix outside St. Athanasius on May 13, 2021. He then burned an American flag and broke a fence in an early-morning tirade the following day, authorities said.

Alaheri has also been slapped with a federal charge of arson in connection to a fire deliberately set at a Borough Park yeshiva building that also houses a synagogue.

At the time of the attack on the crucifix, Msgr. David Cassato, then the pastor of St. Athanasius Church, said, “I’m glad he’s been caught. That means he’s off the streets.”

Msgr. Cassato said the suspect should be forgiven for his actions but that parishioners will never forget what happened — nor should they.

“He obviously needs mental help. No one in their right mind would do something like this. So for that, you have to forgive,” he said. “You want him to get the help he needs. But you don’t forgive and forget. What happened caused a lot of hurt.”