More than one third of the New Testament is devoted to DOCTRINE -  the doctrine of Jesus Christ - BIBLICAL doctrine.
Why would God have allowed so much exhortation to correct doctrine if it weren't that important?
It is apparent that we are living in the "end-times" as prophesied in the Bible and that the
prophesied end-time APOSTASY (falling away from faith) is widespread in the church of today...

2 Thess 2:1 - Concerning the coming of our Lord Jesus Christ and our being gathered to him, we ask you, brothers, 2 not to become easily unsettled or alarmed by some prophecy, report or letter supposed to have come from us, saying that the day of the Lord has already come. 3 Don’t let anyone deceive you in any way, for that day will not come until the apostasy occurs and the man of lawlessness is revealed, the man doomed to destruction. 4 He will oppose and will exalt himself over everything that is called God or is worshipped, so that he sets himself up in God’s temple, proclaiming himself to be God.

The saddening consequences of this is that many 'Christians' are totally unaware that THEY are part of this Apostasy and that they are being misled on how absolutely critical Biblical doctrine is.

Be ye not deceived
Throughout the Bible, the warnings about false prophets and teachers abound and are clear.
In fact, the Bible talks about Deception 93 times in the Bible and more specifically mentions Deception concerning Doctrine 35 times - DO NOT BE DECEIVED! How is it then, that there are so many who blatantly ignore these warnings?!

The Traditions of men

Much of Jesus' own teachings was on correction and the importance of correct doctrine, without which, we are warned over and over again of God's impending judgement. Many are being led astray by following the traditions of men: Matt 15:3 - 7 You hypocrites! Isaiah was right when he prophesied about you: 8 "‘These people honor me with their lips, but their hearts are far from me. 9 They worship me in vain; their teachings are but rules taught by men.’", and the unscriptural teachings of ignorant clergy. Many "Christian" churches have practically turned their backs on the Bible, teaching as their doctrines the precepts of men; effectively turning those true bible-believing churches (who believe in the  the Bible as their sole authority) into members of a "cult".  Nowadays, when one quotes scripture to illustrate and prove correct doctrine, one is shunned as "unloving', 'over-the-top', 'fanatical', 'extremist' etc etc. However, the Word of God is clear; and those who preach comfortable Christianity, the Word-Faith message, a prosperity gospel, Dominion theology, Kingdom now theology, experiential theology, ecumenism and the Unity of the New Age Movement will all have to answer to God for leading their flock astray. Anything that adds traditions of men to Scripture violates this basic fundamental warning, including christmas, easter, lent, the mass (transubstantiation) and psychology, to name but a few.

Matt 15:9 - But in vain do they worship me, Teaching as their doctrines the precepts of men
Any Catholic need just read a few pages of the Catholic Catechism (Laws of Catholicism) and compare them to the plain teachings of the Bible to see how great is Satan's deceptive control of this church. This deception is now practically complete

It is prophesied about many "Christians":  
Matt 7: 21 "Not everyone who says to me, ‘Lord, Lord,’ will enter the kingdom of heaven, but only he who does the will of my Father who is in heaven. 22 Many will say to me on that day, ‘Lord, Lord, did we not prophesy in your name, and in your name drive out demons and perform many miracles?23 Then I will tell them plainly, ‘I never knew you. Away from me, you evildoers!
These are super-spiritual people!! Able to perform miracles "in the name of Jesus"! Why does God turn them away? Because their miracles are based on a false gospel and traditions of men.

We are called to test what man says - don't plainly accept what a preacher says - go to your Bible and "see if it is so" - and if it's not in the Bible, we are commanded by God to reject it, expose it and "be ye separate"! Beware of false prophets. Just because a preacher is ordained (by fallen man!), doesn't mean you have to accept what he says... test it by our final and ONLY authority - The Word of God. 
I John 4:1, "Beloved, believe not every spirit, but trv the spirits, whether they be of God; because many false prophets are gone out into the world."

Why is DOCTRINE so important?
Without correct doctrine, it is impossible to have correct conduct!

Jesus suddenly started talking in parables instead of simple preaching. Why? Because too many didn't want to receive in their hearts what He was saying. Everyone could physically hear Him, but only those who really wanted to hear the Truth could understand. And so it is today... 
He that hath an ear, let him hear

We have no excuse.