Gospel doctrine matters because the good news is so rich and wonderful that it must be opened like a treasure chest, and all its treasures brought out for the enjoyment of the world. Doctrine is the description of these treasures. Doctrine describes their true value and why they are so valuable. Doctrine guards the diamonds of the gospel from being discarded as mere crystals. Doctrine protects the treasures of the gospel from the pirates who don’t like the diamonds but make their living trading them for other stones. Doctrine polishes the old gems buried at the bottom of the chest. It puts the jewels of the gospel truth in order on the scarlet tapestry of history so each is seen in its most beautiful place.
And all the while, doctrine does this with its head bowed in wonder that it should be allowed to touch the things of God. It whispers praise and thanks as it deals with the diamonds of the King. Its fingers tremble at the cost of what it handles. Prayers ascend for help, lest any stone be minimized or misplaced. And on its knees gospel doctrine knows it serves the herald. The gospel is not mainly about being explained. Explanation is necessary, but it is not primary. A love letter must be intelligible, but grammar and logic are not the point. Love is the point. The gospel is good news. Doctrine serves that. It serves the one whose feet are bruised (and beautiful!) from walking to the un-reached places with news: “Come, listen to the news of God! Listen to what God has done! Listen! Understand! Bow! Believe!
– John Piper, from God is the Gospel