Spain's Radio Priest, Fr. Venancio Marcos, 0.M.I., has received his 2.300th convert since he began a weekly broadcast in 1944. His talks have brought him about 6,000 visitors. Fr. Marcos first began to reach prospective converts through the cinema. He wrote film scripts, became a technician and took the part of a priest in several films. Now. however, he says, his most important role in the film world is as a confessor and spiritual adviser to actors.