Is it sinful to study world religions?
I don’t think so. Just don’t practice it. A point that I think most Christians mistakenly believe is that just becasue Christianity is the true religion it means all the other religions are completly wrong. But this is false because there is at least one small seed of truth in all (or almost all) religions. I think a Christian could actually learn some useful things from Buddhism. He needs only be careful to make sure that waht he learns matches up with the Bible before he practices it.