Admittedly, several pastors confessed to me that they do not know much about the world's religions, but decide to teach on them anyways because, "my parishioners are asking me to." Granted, you, as a pastor or teacher, are in a tough place when people ask you to lead a study in an area you feel you know little about. I feel for you. But then there are other pastors who took one class on world religions, watched one documentary, or read one book and decide, "My people need to know this!" and like a crusader gallivanting off to slay the pagan hordes they announce a study to equip their congregants for the spiritual battle at hand. #Facepalm. Maybe you are the former, maybe you're the latter. Either way, you aren't an expert — I implore you to stop acting like one.
Nonetheless, I feel for you. The problem is that we pastors and teachers are expected to be weekly experts on a wide variety of topics. Every Sunday a pastor is meant to churn out a sermon wherein he/she expounds on a relevant topic from a deep knowledge of the biblical text. People listen to the pastor as if he/she is an authority on the given topic (marriage, parenting, politics, etc.). While most pastors (certainly not all) are adept at interpreting Scripture, they are not mavens in every field. It's unfair to expect them to be an expert on everything — especially religions they were not trained in. Too often we pressure them to act as if they are. Likewise, teachers and educators are expected to cover a broad range of topics week-in and week-out, even if their knowledge on some of these topics is exhausted within the confines of the text they use to teach. This problem becomes paramount in teaching on world religions.
With untrained teachers and unqualified pastors diving head first into a study where they are presumed to be specialists, but are effectively faking even basic facility, what most world religion Bible studies become are cesspools of collective religious ignorance not classrooms prepped for increased religious literacy.
Sometimes, in an effort to sidestep an educator's insufficiency for the task, an ex-member testimony is favored. Oh Lord have mercy, this is even worse. Certainly, ex-members have a voice to bring to the table and their perspective is a valuable one to appreciate in our study of religion. But it is only one voice and an extremely biased one at that. Ex-members are ex-members for a reason. While they may not "have an axe to grind" they will most definitely present a prejudiced perspective on a religion they now eschew.
Imagine this -- an atheist meet-up group wants to learn more about Christianity. To do so, they bring in a former evangelical who no longer believes in God to talk about their former faith. Would you, as a Christian, say that the atheists in that group necessarily got a fair picture of Christianity? Would you want them to perhaps balance out their learning with some supplementary teaching or a current member's testimony? If not, you should. Relying on ex-member testimonies or teaching is a sure way to get a skewed impression of a world religion.
So, how do we fix this? Three ways:
The fix: Get an education. Take a class, keep reading, enroll in a master's program. Become the expert you are pretending to be. Even a few classes on one religion will equip you to better teach that topic. However, do not think that taking one intro class on world religions or reading one book is enough. Dive deep into one religion before you endeavor to teach it. Enjoy that process? Keep going deeper or expand your knowledge to include other religions. Repeat as necessary.
The fix: Study in the presence, or even under, the "religious other." While I do not like the fertile terrain for prejudice that "othering" a people group creates, the reality is that most Christians feel that Muslims and Mormons, Jews, Jains, and Jedis are "the religious other." They feel uncomfortable talking about these other faiths in the presence of "the other" (cue creepy sci-fi music here). So, they round up the wagons, close the parish hall doors, and "study" them from the safety of their own sanctuaries. As an educator, your task is to bust those doors down and make the learning environment an uncomfortable one. Bring in a Muslim to team-teach on Islam, invite an atheist to present their non-religious ways, visit a local mosque, temple, or place of worship to engage in experiential education, make your study public, or at the very least ask a Buddhist to sit in on your teaching to call you out or offer further food for thought. Yeah, it will be awkward, unsettling, and a bit "weird," but that's a good thing. In that environment learning is probably going to take place on all sides.
The fix: Bring in the experts. f all else fails, ask the experts. Bring in a local professor or your denomination's resident religious scholar, anthropologist, or sociologist. As mentioned before, bring in a Buddhist monk to share their practice, an imam to elucidate their beliefs, etc. Shameless plug: invite me to come and speak. While I can't speak to EVERY religion with expertise, I can at least point you in the right direction or start you off with the right tools/perspective.
2) The category of "world religions" is problematic anyways
Even if a pastor/teacher is schooled in the ways of the world's religions, what is a "world religion?" Most studies pick out a few heavy hitters among the sundry spiritualities that are held and practiced around the globe. There are some usual suspects that pop up in almost every world religion study. Here's an example from the table of contents of a self-titled "world religion Bible study" curriculum: Christianity, Islam, Hinduism, Buddhism, Judaism, Confucianism, Bahai Faith, Spirit Religions, Atheism, New Age Movement, and others. This is a generous list. Another "world religion" study I saw recently (at a Lutheran church) sought to teach the following: Catholicism, Islam, Anglicanism, Pentecostalism, Buddhism.... Yikes.