Are they useful? I always thought the Bible is sufficient.
We have a wealth of excellent, sound, Bible knowledge as the result of many godly, faithful, Spirit-filled men and women who walked with God, listened to the Holy Spirit within them, and who have received excellent insights into Scripture. These are those who have been called as Teachers in the Body of Christ. This is not a man-made ministry, but it is one of the five-fold ministries listed in Ephesians. Therefore it is a ministry instituted by the Holy Spirit to build up the body of Christ.
I am not talking about legalistic, or liberal teachers who are in theological institutions and who do not teach sound Scriptural doctrine. I am talking about those teachers in Bible Colleges and Seminaries who are teaching sound, Scriptural doctrine to give people a solid foundation in the gospel and the Scriptures.
All the false teachers who started false cults maintained that their interpretation of the Bible was correct and they didn't need anyone else to teach them. Consequently, they went into error and taught doctrines that were doctrines of devils instead of sound Scriptural doctrine. We see this in the mainstream Charismatic movement where kundalini manifestations, "voices", "new" revelation, guaranteed healing, prosperity are taught and the gospel is ignored. These are men and women who have said they don't need theological training and qualifications because their personal interpretation is superior. This attitude comes from an unteachable spirit.
I praise God for the many books I have read, and the many messages I have heard from godly men, that have given me a sound foundation in the gospel through the insights they have shared. I also praise God for the three year journey that I took to get my M.Div, after 45 years in the faith. I thought I knew the Bible inside out, but the discipline of doing the study shows me how much I didn't know and what more I had, and still have, to learn.
Pride is a deceiving thing. People can have pride in their own good works, that they think that they can work their way to heaven. Others have pride of grace where God's work of grace in them has made them someone special. Others have pride in their level of holiness where they think they are holier than the common herd. And then there is pride in our ability to know how to interpret Scripture, and so we ignore the teaching of godly men and women to give us insight into the ways of God through Scripture.
And pride puts one on the highway to hell, rather than on the narrow way to heaven.