This means that everything we believe about everything the Bible teaches matters. Not only that, but since we live out of our beliefs, every behavior of ours that is addressed in the Bible matters. Yes, we are saved by grace and justified by faith in Christ's works on our behalf. Yet the fact remains that the Christian life is one of radical obedience in response to all Christ is, all He has done, and all He has promised to do.
There are two things that prevent people from understanding and truly believing that Doctrine Matters...
This is the friendlier of the two detractors, because good intentions are, well...good. It at least means we are sincere and that's gotta' count for something. Right? Well despite what many people want to believe, it is actually possible to worship (the right) God wrongly - in a way He does not accept. It is also possible to read, and even teach, the Bible and yet be mislead to the point that you think you know God, when you truly do not. In either case, neither good intentions nor sincerity excuse us. It is a dangerous thing to depend solely on our "heart being right before the Lord." Even our intentions and our conscience must be informed by the objective words of God in the Bible. While some are tragically mistaken about their status as Christians, there are many true disciples of Christ who have become content know only what they need to know in order to be saved (mininmalist). They brush their lack of holiness and serious study of Scripture under the rug of their positive feelings toward God without considering seriously the practical implications of the lordship of Jesus. They do not understand that while there is no condemnation in Christ and while His yoke is in deed easy, we are now free as his bond-slaves to be holy by living according to His commands. We are not to trust in our good intentions or allow them to create an artificial contentment with growth in God and His Word. Let's look intently into His Word in order to best please God.
This is the sneaky and deceitful detractor from believing that doctrine matters. Despite the behavior of many professing Christians, grace is anything but cheap. Sadly, many will claim Christ with their words and through certain public behaviors, but never have a real encounter with God's grace in repentance that leads to works. The wide-spread acceptance of cheap grace and easy-believe-ism among the Church has made it difficult to separate the professing and the truly possessing. True saving faith will produce fruit (good works) and this ought to be a warning for many are far too comfortable to not take knowing and obeying God's Word seriously. While our salvation (by grace, through faith) is a gift, we should respond in obedience by offering our entire life as a living sacrifice. Scripture makes it clear to us that we are not our own, but are slaves/bond-slaves of God, bought with the high price of Christ's own blood. Therefore, we desire to do everything (not just barely what is needed to get by) precisely (not just generally or the way we prefer) how He commands in His Word. We are not free to sin, to bask lazily in our freedom in Christ, or to adhere loosely to His commands as though they are divine suggestions. Since, Christ has paid our debt for all of our sins in full, the only response is complete obedience. This costly grace is worth giving our lives to worship and know God accurately according to how He has revealed Himself in Scripture.
The Bible has the authority to inform our beliefs and instruct our behaviors, and we can do neither apart from sound doctrine.