It is the consensus of evangelical New Testament scholars that fully developed Gnosticism, such as developed in the late 2nd century A.D., was a syncretism between some elements of New Testament Christianity, pagan mythology and occult practices, and the secular philosophy of Neo-Platonism.
Most pastors engaged in New Testament study as part of their preaching and teaching ministry may not be familiar with Neo-Platonism. It was in fact the primary secular philosophy in the New Testament world, along with the Stoics and Epicureans whom Paul encountered in Athens. In addition, most pastors may not have access to a translation of the primary work of Neo-Platonism, the Enneads, or have the facility in ancient Greek to read through the original text (a nice PDF is available online) or the time to do much study in Neo-Platonism.
Enter Bertrand Russell, the well known secular and atheist philosopher, to the rescue. His work, A History of Western Philosophy, has a very good summary of Neo-Platonism. His discussion is excellent, and he relies not only on the best history of ancient philosophy available at the time but also seems to have read through the significant works in the original language for himself.
An understanding of Neo-Platonism can be helpful also in understanding Paul’s warnings against the mixture of popularized philosophy and pagan and Jewish religious practices in the letter to the Colossians. It also helps to highlight the reasonableness of the traditional views, from the early church fathers Ignatius, Irenaeus and Hippolytus onward, that Gnosticism represented a heretical departure from early Christianity, and not a legitimate variation from the period of the New Testament, which is the view presented in The DaVinci Code.
Russell’s History of Western Philosophy does make a good addition to the standard evangelical works which document the history of philosophy, such as that of Colin Brown. It can usually be found in used book stores and bargain book displays as well. His style is readable, the work is well documented, and he gives generally sound presentations and critiques of various philosophical positions, notwithstanding his atheism and disparagement of Christianity. I don’t think that it’s a work that a pastor need fear would undermine his faith, but rather might aid his understanding where his preaching and teaching ministry may need to address issues from secular and popularized philosophy.