Doctrine is wonderful. I love doctrine. Doctrine just means "teaching." But it's so much more than that also. Doctrine saves, we are delivered by doctrine!
But thanks be to God, that you who were once slaves of sin have become obedient from the heart to the standard of teaching to which you were committed, (Romans 6:17).
You see the words 'that standard of teaching'. The KJV uses the word doctrine. You were delivered by doctrine. God's teaching saved you. Its usefulness and importance continues after salvation, for it edifies you, strengthens you, grows you, protects you, and more. (2 Tim. 2:24–26; Titus 1:9-11, John 17:17, &etc.).
Here is Scott Swain at Ligonier Ministries to explain:
What is doctrine? In its basic sense, doctrine is any sort of teaching. The Bible, for example, talks about the teachings of men (Mark 7:7–8), the teachings of demons (1 Tim. 4:1; Rev. 2:24), and the teachings of God (John 6:45; 1 Thess. 4:9; 1 John 2:27). Here, we are concerned with divine teaching, the teaching of God. According to one definition, doctrine is teaching from God about God that directs us to the glory of God. This definition provides a helpful anatomy of sound doctrine, identifying doctrine’s source, object, and ultimate end. We will consider these elements of sound doctrine.We are obedient to the Person of Jesus of course, but we are also delivered by doctrine and we are obedient to the teaching of Jesus.
True doctrine becomes your protection. Doctrine, understood, begins to build your convictions. Convictions become your protection. If you have few convictions, you are very vulnerable. The more sound doctrine you know, the more you move from being a spiritual child to a young man." John MacArthur, The Master's University sermon, Delivered by Doctrine.Doctrine as a word and a spiritual discipline has become tarnished of late. This essay is to try and bring the beauty of doctrine to its rightful place in the Christian life and mind. Here are a few more resources for you, in addition to the links already shared in this essay.
What is doctrine? - Got Questions
What is essential Christian doctrine? - Christian Research Institute
Essential Doctrines of the Christian Faith - Christian Apologetics and Research Ministry