Sometimes people wrongly assume one of these two is optional. For some, if they believe the right things, they think it doesn't matter that they exhibit unbiblical behavior. For others, as long as they treat people well, they think it is insignificant what they believe.
However, the New Testament letter of 3 John helps us see how essential it is to both know the truth (beliefs) and treat people in a way that honors Christ (behavior).
In 3 John, we are introduced to four men. There is Gaius, the faithful servant; Diotrephes, the arrogant opponent; Demetrius, the proven example; and John, the beloved elder.
The situation is clear. Diotrephes had a place of leadership in the church, although his behavior was anything other than Christian. We read that Diotrephes was a man "who lives to put himself first." What a terrible reputation. This kind of language is only found elsewhere in the New Testament in Colossians 1:18, where it says of Jesus, "He is the beginning, the firstborn from the dead, that in everything he might be preeminent."
Jesus alone is to be first place. Yet, Diotrephes longed to hold the position that rightfully and only belongs to the Lord Jesus Christ. And the apostle John calls him out on it. It is worth noting the sins of Diotrephes that Scripture highlights:
He was full of pride
He was a gossip
He did not show hospitality
He sought to prevent others from showing hospitality
He wrongfully tried to excommunicate others
He rejected authority
Diotrephes is hardly a model worth emulating!
But did you notice that John does not correct his beliefs? It is not that doctrine is unimportant. Scripture is clear on that! But John's threat of confrontation (3 John 10) is a result of the behavior of Diotrephes. It doesn't seem that Diotrephes was necessarily mistaken about the deity of Christ, the veracity of the resurrection, or the reality of heaven and hell. Yet, whatever he believed (or would say he believed) did not translate into godly living. And because it didn't, Scripture rebukes him strongly.
There are untold numbers of people who enter churches every single Lord's Day. They sing the hymns, know the stories, serve on ministry teams, and hold places of leadership. If you quizzed them on their doctrine, they would excel. However, when it comes to how they treat people, fight against pride, and walk in the truth they claim to believe, they come up wanting. Often, people like this excuse away their disobedience by stating their correct beliefs (which, of course, are essential!) are exemplary. But Scripture does not say correct belief negates the need for proper behavior. Neither does God's Word teach us that right behavior makes correct belief unnecessary.
Instead, the clear teaching of 3 John is that true Christians are those who believe the truth and behave according to it (like Gaius, Demetrius, and John).
Further, 3 John contains only two imperatives for us to follow. We are told in verse 11 to "imitate good" and in verse 15 to "greet the friends, each by name." Yes, we must know the truth and adhere to sound doctrine. But if we do, it should be clear that our right belief is translated to the right behavior. And what are we told to do in 3 John to show our sincere trust in the Lord? Do good (verse 11) and be kind (verse 15). It is not complicated. Our right doctrine, if truly believed in faith, will produce within us the right behavior that makes it all the way down to how we greet each other and speak to one another.
Let it be true of us that we are people who "are walking in the truth" (3 John 4), living as "fellow workers in the truth" (3 John 8), and that we "do not imitate evil but imitate good" (3 John 11). Let's believe right, behave right, and in so doing, demonstrate that we truly belong to Jesus!