Among other things (not having to do with agnosticism), I purchased a lecture series by Bart D. Ehrman a while back which is quite fascinating. If you’re not familiar with his works, I suggest looking up his debates with Christian theologians on YouTube. His logic-based approach to refuting Christian orthodoxy is riveting. You might also get a lot of mileage out of Rabbi Tovia Singer’s “Let’s Get Biblical” lecture series which quite handily refutes almost every aspect of Christian apología imaginable.

Generally, according to my own experience, the quality of instruction offered by the Great Courses lectures is very satisfying.

I always receive GC mailings, but I have yet to actually investigate them. I must admit that I live less than a mile walk from a university library and that always fulfills my needs cheaply. I am going to see if they have any GC material available.