Christensen was detained during a police raid on a local prayer meeting he was leading in May 2017.
In a report past year, Human Rights Watch accused the Russian authorities of carrying out a "sweeping campaign" of harassment and persecution against the movement.
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.
Reuters interviewed Christensen in his jail cell during breaks in the trial.
Christensen had pleaded not guilty in the case, but the court found him guilty.
His wife, Irina Christensen, told reporters, "The same thing could happen to any of us".
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Now, almost 100 members of the group face charges in Russian Federation and more than 20 of them are in jail awaiting trial, according to the AFP.
Authorities in 2017 amended an existing anti-extremism law to mention Jehovah's Witnesses specifically, ordering their dissolution in Russian Federation.
Kremlin spokesman Dmitry Peskov said that there had been reasons for Christensen's arrest, but that he was unaware of the details of the case.
Christensen's lawyer Anton Bogdanov said that he will likely file an appeal within 10 days after discussing the matter with his client.
"Deeply concerned by the sentencing of Dennis Christensen".
Denmark's Foreign Minister Anders Samuelsen said he was "deeply concerned" and called on Russian Federation "to respect freedom of religion". "It's shocking that in post-Soviet Russia authorities are putting people through the ordeal of a criminal investigation and prison for nothing more than peacefully practicing their faith".
"An innocent man who did not commit any real crime was convicted", said Yaroslav Sivulsky, a representative of the European Association of Jehovah's Witnesses. "It is sad that reading the Bible, preaching, and living a moral way of life is again a criminal offense in Russian Federation".