When I try pray at the altar I get a message: 'Repent your crimes wicked one!'
What should I do?

Do you have a bounty if so pay it.

Make sure your fame is higher than your infamy. Have you been doing crimes or working for thieves guild or dark brotherhood? You get infamy by completing their quests. To lower your infamy, walk the Pilgrims Way, or do "good" quests. More information about infamy can be found http://www.uesp.net/wiki/Oblivion:Infamy. More info on famy, http://uesp.net/wiki/Oblivion:Famy.

Kill more people, mainly priests and zealots. That should improve your situation overall.

You should never, ever listen to pdesbois.

If you have knights of the nine you can do pilgrimmage. If you have vile lair you can use that shrine as well.

I keep imagining the unsuspecting wictim of my wise suggestions *trying them out*, in good faith - even though they are pretty clearly hoax. :chaos: