I found the easest topic to do for my CBA was to write about the different parables and miracles. There should be information in your textbook about them. being honest, it was the simplest thing ever. if you wanted, you could write about the different types of sacraments in Christianity. With each sacrament include, a defiinition of that sacrament, the action (what happens), the symbol (Communion-bread annd wine (body and blood of Jesus)) and some of the specific words related to that sacrament (Communion-"This is my body").
Hope this helps.