The following is adapted from this book (of which I happen to be the rights-holder). For anyone interested, the full text of the book is available both online in Google Books, as well as for download as PDF from the same site (simply hover over the EBOOK link on the left and click PDF to download).
So Jesus was saying to those Jews who had believed Him, “If you continue in My word, then you are truly disciples of Mine; and you will know the truth, and the truth will make you free.”
John 8:31-32, NASB, emphasis added
The "Words in Red" are Jesus' instructions for all believers. While some would claim otherwise, perhaps out of preference for the 'Pauline Revelation', scripture is actually quite clear about this topic.
The argument otherwise is first of all built on a faulty premise. It is assumed by some that the Gospels are merely a history book, and not teaching. However, one need only look to the end of the Gospel of Matthew to find otherwise.
Then Jesus came to them and said, All authority in heaven and on earth has been given to me. Therefore go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, and teaching them to obey everything I have commanded you. And surely I am with you always, to the very end of the age.
Matthew 28:18-20, NIV, emphasis added.
Jesus' instructions were to make disciples by teaching all of His commands. Further, in the beginning of the book of Acts, immediately after Pentecost in chapter 2, we read,
They devoted themselves to the apostles’ teaching and to fellowship, to the breaking of bread and to prayer.
Acts 2:42, NIV, emphasis added
Of the four things that was the focus of the first church, the first of them was the apostle's teaching. As this was the first months and years after Jesus' death, resurrection, and ascension, the disciples would have had nothing to teach except what Jesus had taught them. The gist of this, what they deemed most important, is what we have recorded in the four Gospels by way of their recording.
Further evidence that this was the intent of the writing of the Gospels can be found in the beginning of Luke's Gospel.
Inasmuch as many have undertaken to compile an account of the things accomplished among us, just as they were handed down to us by those who from the beginning were eyewitnesses and servants of the word, it seemed fitting for me as well, having investigated everything carefully from the beginning, to write it out for you in consecutive order, most excellent Theophilus; so that you may know the exact truth about the things you have been taught.
Luke 1:1-4, NASB, emphasis added
What is contained within the Gospels, then, was for the instruction of new disciples. The writing and subsequent learning, teaching, and reading of the Gospels is the fulfillment of the command in Matthew 28:20, to teach what Jesus had commanded them.
Confirmation of Jesus' Words in the Epistles
Paul, of course, was not one of the original apostles. Being a devout Jew, he most likely heard of Christ, but was not a believer. However, after his salvation experience, he clearly learned more of Him. While some claim that the 'Pauline Revelation' supercedes the Gospels Words, Paul wrote differently.
If anyone advocates a different doctrine and does not agree with sound words, those of our Lord Jesus Christ, and with the doctrine conforming to godliness, he is conceited and understands nothing; ...
1 Timothy 6:3-4, NASB, portion, emphasis added
If this was Paul's views of the teachings of Christ, those who claim a 'Pauline Revelation' should agree with his assessment of Jesus' Words. But, Paul was not alone in this conclusion. John wrote the same in his second Epistle.
Anyone who runs ahead and does not continue in the teaching of Christ does not have God; whoever continues in the teaching has both the Father and the Son. If anyone comes to you and does not bring this teaching, do not take them into your house or welcome them. Anyone who welcomes them shares in their wicked work.
2 John 1:9-11 NIV
The witness from the epistles agrees. But, what of Jesus Himself?
The Words of Christ
Heaven and earth will pass away, but my words will never pass away.
Matthew 24:35 NIV
Jesus was the Truth (John 14:6). Jesus pointed to His own words time and again, indicating their importance and immutability. Indeed, at one point, Jesus said it was His Words themselves that would judge those hearing them (John 12:48).
Therefore everyone who hears these words of mine and puts them into practice is like a wise man who built his house on the rock. The rain came down, the streams rose, and the winds blew and beat against that house; yet it did not fall, because it had its foundation on the rock. But everyone who hears these words of mine and does not put them into practice is like a foolish man who built his house on sand. The rain came down, the streams rose, and the winds blew and beat against that house, and it fell with a great crash.
Matthew 7:24-27 NIV, emphasis added
What does it mean to be someone's disciple? It means you do what they say. If Jesus' words were not binding on the church today, it would follow that we would not Christ's disciple--we would be the disciples of the one we did obey.
Heaven and earth shall pass away: but my words shall not pass away.
Mark 13:31 KJV
Here, and above, Jesus Himself said His Words would never pass away.
Verily, verily, I say unto you, he that heareth my word, and believeth on him that sent me, hath everlasting life, and shall not come into condemnation; but is passed from death unto life.
John 5:24 KJV
Jesus linked salvation to two things. The first was hearing His Words, and the second was believing on the Father. As hearing His Word is held as part of salvation, it seem highly unlikely that they would no longer be relevant to the world.
Then said Jesus unto them again, Verily, verily, I say unto you, I am the door of the sheep. All that ever came before me are thieves and robbers: but the sheep did not hear them. I am the door: by me if any man enter in, he shall be saved, and shall go in and out, and find pasture. The thief cometh not, but for to steal, and to kill, and to destroy: I am come that they might have life, and that they might have it more abundantly.
John 10:7-10 KJV
Consider what it means to come in through Christ. Yes, it is salvation, through faith alone. Yet, He was what He taught— He was The Word Incarnate! You cannot “enter through Christ” without obeying what He said. All who try to enter another way are thieves and robbers.
If ye love me, keep my commandments.
John 14:15 NIV
Being observed, that it is impossible to keep something you don't have.
He who believes in the Son has eternal life; but he who does not obey the Son will not see life, but the wrath of God abides on him.
John 3:36 NASB, emphasis added
Here, belief is contrasted with obedience. Who is this Christ, the one John calls The Word? He is what He taught. If you want a portrait of Christ, study the Sermon on the Mount. If you love Him, you love it, and vice versa.
Whoever has my commands and obeys them, he is the one who loves me. He who loves me will be loved by my Father, and I too will love him and show myself to him.
John 14:21 NIV
What are the “my commands” of this verse? It is what He was going around teaching, of course. This is the one that loves Him, the one who has and keeps these commands. Thinking in retrospect, if we come up with explanations of why the rest of His Words do not apply to us, shouldn't we come up with an explanation of why we think this one still should?
If ye abide in me, and my words abide in you, ye shall ask what ye will, and it shall be done unto you.
John 15:7 KJV, emphasis added
The key to answered prayer is keeping the Words of Jesus within you. If you keep them, meditate on them, abide in them, you will have your prayers answered, whatever you ask.
If you keep My commandments, you will abide in My love; just as I have kept My Father's commandments and abide in His love.
John 15:10 NASB, emphasis added
Again, if we keep His commands, we abide continually in His love, even as He abode in His Father's love. This is the great key, that we hear His Words, not words of the old letter of the law, but the Words of the Spirit of life in Christ Jesus, living by a new law, the law of His Spirit. But, these are His Words, which are Spirit, and which are Life (John 6:63).
Then Simon Peter answered him, Lord, to whom shall we go? thou hast the words of eternal life.
John 6:68 KJV, emphasis added
Simon Peter recognized the Words of Jesus. As He said, they are the words of "Eternal Life". Certainly, if another had been intended, we should follow them, but there isn't any.
Let the word of Christ dwell in you richly in all wisdom; teaching and admonishing one another in psalms and hymns and spiritual songs, singing with grace in your hearts to the Lord.
Colossians 3:16 KJV
Paul here again exhorts that the Word of Christ dwell in us, meaning His teachings as represented by what is recorded in the four Gospels. It is not the words of another, but particularly those of Christ. Even as Paul quoted a saying of Jesus in Acts 20:35 a statement that is not found in the Gospels, and as the Apostle John wrote that Jesus did and said many other things than those recorded for us (John 20:30). Paul was very clearly aware of what the Lord Jesus had done and said, and, being commissioned of the Lord to instruct us, exhorted us to obey Christ's teachings.
By Paul's own letters we understand that he understood that his exposition was secondary to the Word of Christ, the living Word. Belief in Jesus, in His Kingdom, in His Way. As depicted in who He was, how He lived, and what He spoke.
If any man teach otherwise, and consent not to wholesome words, even the words of our Lord Jesus Christ, and to the doctrine which is according to godliness;
1 Timothy 6:3 KJV
Quoted above, as well. Paul, even in his letters, exhorts Timothy to the words of Christ, which are the wholesome words. This is the doctrine of the church. Paul merely expounded upon them, even as Jesus said, teachers of the Law instructed in the Kingdom would bring out of their storehouses old treasures as well as new (Matthew 13:52).
And hereby we do know that we know him, if we keep his commandments. He that saith, I know him, and keepeth not his commandments, is a liar, and the truth is not in him. But whoso keepeth his word, in him verily is the love of God perfected: hereby know we that we are in him. He that saith he abideth in him ought himself also so to walk, even as he walked.
1 John 2:3-6 KJV
Over and over, belief, obedience, and love are all connected to God's commands. If anyone says that obedience does not matter, the “apostle of love” differs with him in this verse. And, you cannot obey His Words if you throw them out.
Anyone who goes too far and does not abide in the teaching of Christ, does not have God; the one who abides in the teaching, he has both the Father and the Son.
2 John 1:9 NASB
This was one of Christ's twelve apostles, speaking of the Words of Christ. Are they binding on the church? They are according to the one who leaned upon His breast. Whoever does not abide in them does not have God.
His Words and His Heart are inseparable. Belief and Obedience are two sides of the same cloth. His Words are the Words of the Kingdom, His Words were the Words of Life.