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.

Answer:

It could hardly be said that Jehovah's Witnesses "stood up to Hitler". JWs won't/wouldn't serve in ANY army, not just Hitler's. They were rounded up because they didn't recognize Hitler's authority, just as JWs today don't recognize fully the authority of governments.

In addition to the religious/"racial" groups mentioned above, many Catholics (including priests) went to the camps, some for helping Jews evade the SS.

Addition:

Some people believe the definition of Holocaust should include other groups systematically murdered, including other Christians (including Catholics and Jehovah's Witnesses), Poles, Gypsies, Communists, Homosexuals, Disabled People, and various political oppositions.

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.

Further addition:

Referring to Nazi records would be difficult since most were destroyed or permanently sealed towards the end of the war ( a lack which gives Holocaust deniers traction). The huge number of Russian PoWs who were starved to death in PoW camps alongside (but separate from) other Allied servicemen should also be mentioned. There were certainly more deaths than the 6 million to 6 1/2 million commonly quoted, and there are other issues such as locals essentially starved to death because Nazis stole their food or destroyed crops, right across Europe from France to Russia. Of course the Communists also destroyed crops and food stores and it could be argued that the Soviet toll wasn't too far off that of the Nazis.