The German priest, who is in his early 30s, told the court on Tuesday that the girl was "the love of his life".
"I am very unhappy because of what has happened. At the same time, she is the love of my life," he said. "I did not know that the age of consent was 16 years old in Norway. In my home country of Germany it is 14 years old."
The girl called on the court not to condemn the priest when she was called to give evidence, saying he had not coerced her and that the relationship had only come to light after she made a church confession she had believed was confidential.
"I do not think he has done anything wrong," she said.
When the case first came to light in August, Bernt Eidsvig, the Bishop of Oslo, sought to downplay the relationship, telling the VG newspaper that sexual relations had happened only for "a short period" of "a little over a week".
The girl said she had been horrified when the man was charged following a confession she had made in church.
"I wanted to put this chapter behind me and asked to confess," she told the court. "I was assured that it was 100 percent confidential, but that confidentiality was broken and it had catastrophic effects."
According to Alois Broderson, pastor of St. Paul Church of Bergen, as the girl was not a confirmed Catholic, her confession was not confidential.
"I find it appalling that a girl of fifteen years should be guaranteed confidentiality, only for it not to apply because she is not Catholic," Yngvil Semb Hartmann, the girl's lawyer, told the court.
The priest said that the relationship had developed from an innocent flirtation he began with the girl when he worked in a youth ministry for the Catholic Church in Bergen during the summer.
"After an evening event, she waited for me in the sacristy," he told the court. "She was with me. We talked together and kissed each other. It was the first time we had sexual relations."
The girl then messaged him on Snapchat, asking if he wanted to meet again, after which the relationship developed further.
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"If I had been familiar with Norwegian law in this field, it would not have happened," he said. "When we started the relationship, I knew I broke the laws of the Church. I was not aware that I was also breaking Norwegian law."
The case was the latest sex scandal to hit Norway's Catholic Church.
In 2010, it emerged that Georg Müller, a former Bishop of Trondheim, had sexually abused an altar boy at the start of the 1990s. The Church had been made aware of the incident, but had sought to hide it from the authorities.
The priest faces six months in jail, one of them suspended, if found guilty of abusing the 15-year-old girl.