A favorite synonym of “sound doctrine” is “orthodoxy.” To be orthodox is to conform to what is true, and while there are differences as to what sources contain orthodoxy, we regard the Scriptures as the only source to this end while some of our neighbors would include alongside the Scriptures various creeds and councils—such as the Nicene. Nevertheless, our differences on this point notwithstanding, within the Scriptures is an important key to discerning what is “sound doctrine,” or orthodoxy.
In Titus 2:1, Paul wrote, “But as for you, speak the things which are proper for sound doctrine.” The term “sound” in Greek is also translated regarding those who are physically well as opposed to sick (Luke 5:31; 7:10; 3 John 2). When the prodigal son returned, he was received safe and “sound” (Luke 15:27). All other usages refer to doctrine and not physical health, but there’s a lesson herein: those who are physically healthy are sound, or well, so it may reasonably follow that those who are spiritually healthy are sound too.
Many people denounce doctrine as too divisive. It can be, but it ought not to be. Some people exclaim, “I just want Jesus, don’t give me doctrine.” The fault in this request is that we cannot separate the truths of Jesus from the Person of Jesus, so without sound doctrine, we have no Jesus. John contended with this in his day,
By this you will know the Spirit of God: Every spirit that confesses that Jesus Christ has come in the flesh is of God, and every spirit that does not confess that Jesus Christ has come in the flesh is not of God. And this is the spirit of the Antichrist, which you have heard was coming, and is now already in the world. (1 John 4:2–3)
The way some behave today, they might have then said, “Just give me Jesus, I don’t want doctrine,” but for John, it mattered whether or not Jesus appeared in the flesh as a matter of orthodoxy.
What I’d also note about orthodoxy is that it should lead to orthopraxy—right living. In the so-called pastoral epistles of 1 & 2 Timothy and Titus, the belief was reflected in how one lived. Those who lead sinful lives did not order their lives according to “sound doctrine” (1 Timothy 1:8–11; 6:1–5; 2 Timothy 4:3–5; Titus 2:1–10). When one reads all of these passages, we’re struck by how when sound doctrine is mentioned, behavior should follow. A case in point of violating sound doctrine unrelated to us: the Archbishop of Canterbury—who is the head of the Church of England behind the monarch—refuses to say whether or not homosexuality is a sin. We can see here that the behavior of those does not accord with sound doctrine, and anyone claiming to represent God is to call things what God calls them and exhort others to order their lives around sound doctrine.
The sad thing is that so many are fearful of calling specific actions, lifestyles, or choices sinful, so we dilute orthodoxy to avoid hurting feelings. We can and should be able to call sin, sin and at the same time be compassionate and loving towards those in such sins. After all, many of us were there too at one point.
Sound doctrine, therefore, is spiritual health. When one is healthy, they are active. The spiritually healthy person so formed by sound doctrine then does the things that please God.