The country's Supreme Court ruled in April 2017 that organizations related to the religion are extremist and should be closed down, upholding an original ruling by a court in Oryol Regional Court that the group violated extremism laws.
In the Soviet Union under Stalin, Jehovah's Witnesses were persecuted for their faith, facing arrests and deportation.
The Guardian reports that Christensen was detained by armed police in May 2017 at a prayer meeting in Oryol.
The court therefore charged Christensen with "actively involved in organizational work aimed at continuing the unlawful activities of the banned Orel Jehovah's Witness organization".
Human Rights Watch condemned the sentencing and verdict Wednesday.
Yaroslav Sivulsky, a spokesperson for the Jehovah's Witnesses, told Reuters that they consider the verdict to be unjust. In court, a "secret witness" accused him of being a leader in the city's Jehovah's Witness chapter.
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Reuters interviewed Christensen in his jail cell during breaks in the trial.
Jehovah's Witness leaders lamented Wednesday's verdict as a failure of Russian Federation to uphold legally guaranteed religious freedom.
The powerful Russian Orthodox Church has spoken out against the group, with one Church official branding it a "destructive sect".
"Dennis Christensen has been arrested and prosecuted by the authorities simply for practicing his religion as Jehovah's Witness", said Marie Struthers, Director for Eastern Europe and Central Asia at Amnesty International, in a statement before the sentencing.
Dmitry Peskov, a Kremlin spokesman, said there were clearly reasons for Christiansen's arrest but he was unaware of details.
In December, Putin vowed to look into the persecution of Jehovah's Witnesses, saying the labeling of religious communities as terrorist organizations was "complete nonsense". Anton Bogdanov, his lawyer, said they plan to appeal the verdict 10 days from Wednesday.
"We deeply regret the conviction of Dennis Christensen - an innocent man who did not commit any real crime", Sivulskiy said in a statement. "It is sad that reading the Bible, preaching, and living a moral way of life is again a criminal offense in Russian Federation".