He [an elder or overseer] must hold firmly to the trustworthy message as it has been taught, so that he can encourage others by sound doctrine and refute those who oppose it.
You, however, must teach what is appropriate to sound doctrine.
As I urged you when I went into Macedonia, stay there in Ephesus so that you may command certain people not to teach false doctrines any longer.
For the time will come when people will not put up with sound doctrine. Instead, to suit their own desires, they will gather around them a great number of teachers to say what their itching ears want to hear.
Watch your life and doctrine closely. Persevere in them, because if you do, you will save both yourself and your hearers.
It is this last verse that I really want to focus on. Paul writes to his protégé Timothy, warning him to hold fast to didaskalía—that is instruction and “property applied-teaching.” Given the above directions to teach in accordance with sound doctrine and to be wary of those who will present and seek unsound doctrine, we must ask ourselves two questions. First, how do we determine what is or is not sound doctrine. Second, how do we watch our doctrine closely?
Fortunately, God gives us answers to both questions. Titus 1:9 gives us the short answer to how to analyze doctrine. Paul writes to “hold firmly to trustworthy message as it has been taught” (emphasis added). That message is the gospel. We hold to sound doctrine when we hold to the gospel. In the next chapter, Paul follows up his command to “teach what is appropriate to sound doctrine” with lists of things to teach, suggesting they are consistent with sound doctrine.
John also gives us methods of testing purveyors of doctrine, whether it is good or bad, when he writes, “Anyone who runs ahead and does not continue in the teaching of Christ does not have God.” Furthermore, he instructs, “This is how you can recognize the Spirit of God: Every spirit that acknowledges that Jesus Christ has come in the flesh is from God, but every spirit that does not acknowledge Jesus is not from God.”
The simple test of doctrine is whether or not it agrees with God and His Word. My wife works at a bank, and she tells me that the way tellers at a bank spot counterfeit bills is not by studying counterfeit bills to know what they look like, but by studying genuine bills so that they recognize anything inconsistent with them. Put another way, as the Newsboys once sang, “we know a line is crooked ’cause we know what’s straight.” The way to recognize false doctrine is to be so intimately familiar with true doctrine that anything contrary triggers red flags and alarm claxons.
This dovetails into watching our doctrine closely. Again, Scripture guides us. Paul wrote to the Thessalonians, “So then, brothers and sisters, stand firm and hold fast to the teachings we passed on to you, whether by word of mouth or by letter.” He told Timothy to persevere, that is, to keep on keeping on in his life and doctrine. He instructed the church in Ephesus that “Christ himself gave the apostles, the prophets, the evangelists, the pastors and teachers, to equip his people for works of service,” saying then that the body would be built up, be unified, and would attain to “the whole measure of the fullness of Christ.” Look what comes next: “Then we will no longer be infants, tossed back and forth by the waves, and blown here and there by every wind of teaching and by the cunning and craftiness of people in their deceitful scheming.”
I could go on and on citing New Testament verses and directives because, in essence, the extent of the epistles show us how to watch our doctrine closely. They give us correct doctrine, the plumb bob by which we can tell if something is crooked, and they give us the methods of putting that doctrine into practice, of holding fast to it. Thus Paul's admonition to also "watch your life." The two are intricately linked.
I’ll close by quoting J.I. Packer, which I believe I’ve done before, but it bears repeating: “Doctrinal preaching certainly bores the hypocrites; but it is only doctrinal preaching that will save Christ's sheep.”
Sounds like somebody else we just read . . .
 Titus 1:9
 Titus 2:1
 I Timothy 1:3
 II Timothy 4:3
 I Timothy 4:16
 See Strong’s Concordance
 HELPS Word-studies copyright © 1987, 2011 by Helps Ministries, Inc.
 II John 1:9
 I John 4:2-3
 Newsboys. “Believe.” Step Up to the Microphone, Star Song, 1998.
 II Thessalonians 2:15
 Ephesians 4:11-14