What were most religions in the concentration camps?
The only concentrated group was the Jews,(and it was a massive concentrated effort by the NAZI's) the balance were just whatever the prisoners happened to be.
The only religion to take a firm stand against Hitler was Jehovah's Witnesses, then known as the International Association of Ernest Bible Students. And for this they were persecuted severely, thousands were sent to concentration camps. The SS tried every form of torture and cruel sadistic treatment to get the Bible Students to sign a declaration renouncing their faith and willingness to become a soldier. Many young men who refused to bear arms in Hitler's war were executed for their stand. Hitler was infuriated by the resistance of this small but unyielding group. The camp commander at Sachsenhausen ordered August Dickmann, a young Witness to be executed in the presence of all the prisoners, with Jehovah's Witnesses out front where they would get the full impact. Afterwards he dismissed all but the Witnesses and asked "Who is now ready to sign the Declaration?" Not one of the approximately 400 present responded. Then two stepped forward; not to sign, but to ask that their signatures given a year earlier be annulled. The record of the treatment given those in the Nazi concentration camps to all the various groups- Jews, Gypsies, homosexuals, criminals, and Jehovah's Witnesses is documented fully at the Holocaust Museum in Washington DC.
In addition to the religious/"racial" groups mentioned above, many Catholics (including priests) went to the camps, some for helping Jews evade the SS.
This would bring the real toll to between 11 and 17 million people. We know approximately 6 millions Jews were exterminated, so that indicates that between 5 million and 11 million Atheists, Christians, and Pagans were exterminated. It is would be difficult to break down the exact numbers per religion without referring to the Nazi records.
Concentration camps were camps that the Jews were sent to during the time of the Holocaust. There were four major concentration camps. Most were killed there in torturous ways, forced into physical labor, or forced into sexual slavery. Not only Jews were sent there though also gypsies, homosexuals, just about anyone who was not a Nazi.