Is it considered theft to take religious clips from other YouTube channels that do not own the copyrights and publish them on my channel, and what should I do after having published many of them without permission?

What is prohibited from being published without permission are copyrighted materials whose owners forbid their publication. As for materials that are not known or are predominantly thought to be copyrighted, there is no harm in publishing them without permission. Merely uploading video clips of scholars' lessons and Qur'anic recitations to YouTube does not grant the channel owner ownership of the material, as the rights belong to the scholars or reciters. It is necessary to delete materials that have copyrights and whose owners are known or predominantly thought to forbid their publication.