Go to the Word
The Lord gave us this warning concerning placing man's word over God's. In addition to the physical revelation of God in the body of the Lord Jesus, God has given us the revelation of Him through His written word--the Bible, to be used as a guide through life. He also added an extra benefit in the person of the Holy Spirit to teach and provide understanding of the Word (John 14:26).
Just after giving this warning, the Lord told us to beware of false Christs that will deceive many. He wasn't necessarily warning against anyone pretending to be the very Christ (although we should be aware of such), but of anyone taking His name (Christians) and using it to deceive His children. There are many false prophets, teachers, preachers in the world--not only ones who preach a perverted gospel or disperse doctrines that are unsupported by Scripture, but more commonly, ones who fail to teach and preach the entire Word of God. In my opinion, a half-truth is the same as a lie. In fact, it may be worse than a lie because it gives a person a distorted sense of truth. Just as we love the full flavor of a cold drink with ice, the taste isn't as good when left outside and the ice melts, becoming watered-down and lukewarm. The same applies to the Word of God--the complete Word is better than the watered-down version.
So how do we distinguish the truth from lie? We go to the Word of God. Its teaching is our doctrinal foundation and everything we are taught should be proven and confirmed by the Bible. The Bible says: Study to show yourself approved unto God, a workman need not to be ashamed rightly dividing the word of truth. [2 Timothy 2:15] Your independent study of the Word is the personal foundation while any teaching you sit under simply supports what you should have already studied. Otherwise, if you do not prove what you have heard--either by it bearing witness with what you have previously learned through Scripture, or by proving it afterward, you are in danger of falling prey to the false Christs the Lord warned us about.
The apostle Paul wrote similarly to the Ephesian church: Proving what is acceptable unto the Lord. But all things that are reproved (or exposed) are made manifest by the light. For whatsoever does make manifest is light. [Ephesians 5:10, 13] Paul was referring to the works of darkness, but the same command applies with false teachings and doctrines. The Word is light and our knowledge of it will allow us to discern the truth from false.
Too many Christians are depending on their pastors, Sunday school teachers, and others to feed them the Word of God and consequently, they have fallen into deception. This is what the Lord was warning us about--being knowledgeable of the Word of God through our own study, lest any man deceive us. The Lord has given us teachers, apostles, prophets, pastors, and evangelist to build up the body of Christ and prepare us for the works of the ministry (Ephesians 4:11-12). We all need the Word (through teaching and independent study) to grow--as we need food to live. However, we must be prepared to recognize what is nourishing for our minds, and what is damaging or what keeps us from coming unto the fullness of Christ.
God gave us His Word not only for us to read along with the pastor or teacher, but more importantly, for us to read and study alone (with Him). Jesus said: If you continue in my word, then are you my disciples indeed. And you shall know the truth, and the truth shall make you free. [John 8: 31-32]
