A Catholic priest who had served time for having a sexual relationship with a woman, allegedly while he was meeting with her for spiritual counseling, will get a new trial because his constitutional rights were violated, the Minnesota Court of Appeals ruled on Monday.

Christopher Thomas Wenthe, 46, was convicted last year of one count of third-degree criminal sexual conduct during a religious-advice meeting. He maintained that it was a consensual relationship and that no crime was committed.

Monday, the appellate court ordered a new trial for Wenthe in Ramsey County.

In a two-part decision, the appellate judges upheld a Ramsey County judge's decision that a state law is based on secular standards -- which do not involve religion matters -- and therefore doesn't violate the Establishment Clause, which holds that states can't make laws establishing or preferring a religion.

But the appeals court threw out the priest's conviction on grounds that prosecutors relied too much on church rules in presenting evidence.

"The basis for the criminal case was that he violated his vow of celibacy," said Wenthe's attorney, Paul Engh. "That vow is a church standard and not a secular standard. Moreover, he could not be convicted because in the eyes of the church he was deemed holy. And yet that's what the state argued for his guilt."

Nor could he be convicted, Engh said, "because he was in a position of power under church law," another argument by prosecutors.

In November 2011, jurors convicted Wenthe of sexual conduct "during the course of a meeting" where religious advice or assistance was sought or received in private.

Wenthe didn't dispute the sexual relationship, only that it occurred while he was providing spiritual aid or comfort -- the linchpin distinction in determining clergy sexual misconduct.

Wenthe, of St. Paul, had been newly ordained when the woman, then 21, performed oral sex on him in the rectory of Nativity of Our Lord Catholic Church in St. Paul in 2003. The two had an 18-month relationship.

Appellate judges wrote the conviction "was based on evidence that was excessively entangled in matters of religion." That violates the Establishment Clause under the First Amendment, the opinion said.

The Establishment Clause provides that "Congress shall make no law respecting an establishment of religion." That means the government can't intrude on religious doctrine, nor on a church's internal decisionmaking, Engh said.

Wenthe served time in the workhouse as part of a 57-month sentence, with the rest stayed.

"That's a sad thing, that he spent eight months in jail for a conviction that was infirm," Engh said.

Wenthe was ordered to register as a predatory sexual offender. He remains a Catholic priest on inactive status.

Andrew Eisenzimmer, legal counsel for the Archdiocese of St. Paul and Minneapolis, said the decision didn't surprise him because it was based on religious evidence, which violated the Establishment Clause.

Ramsey County Attorney John Choi expressed disappointment that the jury's verdict was overturned, not only "for the sake of the victim" but also for its negative impact on a 1993 statute.

"This case is the classic circumstance that the Minnesota Legislature intended to criminalize in 1993 when it expanded the criminal sexual conduct laws to include clergy, recognizing their enormous power and influence over those who seek their spiritual guidance and counsel," Choi said.

"Under the statute, the victim's purported consent is not a defense. Clergy, like psychologists, counselors and other professionals listed in the criminal sexual conduct statute, are oftentimes asked to help the vulnerable, not to exploit them."

Dale Carpenter, constitutional law professor at the University of Minnesota Law School, said the opinion could force prosecutors to walk a fine line in trying similar cases.

"This is a narrow decision, but it could actually put prosecutors in difficult positions in future prosecutions because it's obviously relevant that this is a clergy situation, but you can't talk too much about it," he said.

Joy Powell • 651-925-5038 Staff writer Rose French contributed to this report.