If we are going to help someone grow in their Christian faith or study the Bible with someone—whether it is with an unbeliever or a new believer—we are bound to encounter some doctrinal and theological error along the way. Even if one has not given much thought to that possibility, we intuitively know that when we do encounter serious error at some point, it is our duty to lovingly correct the error in such a discipleship relationship. We also know that we don’t want to be overbearing or perceived as constantly waiting to slap wrists with a ruler. So, how do we know when it is important to address matters of biblical truth?
Prioritize by the Person’s Situation
For the unbeliever, we are not dealing with a new creature in Christ that is aided by the indwelling of God the Holy Spirit. We can’t expect them to exhibit Christian behavior or proclaim sound Christian doctrine—and we shouldn’t place that burden on them. We really should focus on communicating the gospel message to them. To spend time talking about what the Bible has to say about church discipline, for example, would be unwise and unfruitful.
Prioritize by the Impact of the Error
For the believer, we should focus on correcting error that is the most foundational. To help get you started on which issues to prioritize, four categories are offered to grade the impact of the error: essentials (a belief necessary for salvation; to get an essential wrong is to distort the gospel), practical near-essentials (a belief that is not essential for salvation but that a Christian should nonetheless believe in order to have a healthy faith), conviction (a belief where unity is necessary for Christians to work together in the same local church), and preference (a matter upon which Christians in the same congregation may disagree while maintaining local church unity).  Address essentials and practical near-essentials first. If you go to the same church or to churches that have a similar doctrinal statement, it would make sense to spend time on convictions. Don’t spend much time (if at all) on preferences.
Essentials – A belief necessary for salvation. To get an essential wrong is to distort the gospel.
It is not enjoyable to correct anyone or point out error. However, we are not doing anyone any favors by ignoring a situation where someone gets an essential wrong. It is our job to lovingly address these things as they come up (in a non-confrontational way). My recommendation would be to take them to a passage of Scripture that makes it clear. For a new Christian, it will probably be a really short conversation that results in something like, “I didn’t know that.” For a new believer, instead of having this conversation regularly, it might be a good idea to go through a quick introductory study (like Start Strong: A Guide for New and Growing Believers) with them before starting a study of a book of the Bible.
Two Types of Essentials: Must Believe and Must Not Deny
Michael Wittmer notes that what one must believe to be genuinely saved is incredibly simple: “In the book of Acts, the bare minimum that a person must know and believe to be saved was that he was a sinner and that Jesus saved him from his sin. As Paul told the Philippian jailer, ‘Believe in the Lord Jesus and you will be saved’ (Acts 16:29-31; cf. 10:43).”
Must Not Deny
Wittmer further points out that while it is not necessary for a believer to initially understand and confess faith in every essential doctrine, there are beliefs that believers must not deny as they grow in their faith: “But any thinking convert will inquire further about this Jesus. While he may not know much more at the point of conversion than Jesus is the Lord who has saved him, he will quickly learn about Jesus’ life, death, resurrection, deity and humanity, and relation to the other two members of the Trinity. Anyone who rejects these core doctrines should fear for their soul.
According to the Athanasian Creed, whoever does not believe in the Trinity and the two natures of Jesus is damned. However, since it seems possible for a child to come to faith without knowing much about the Trinity or the hypostatic union (this is likely not the place where most parents begin), I take the Creed’s warning in a more benign way—that we do not need to know and believe in the Trinity and two natures of Christ to be saved, but that anyone who knowingly rejects them cannot be saved.”
Practical Near-Essentials – A belief that is not essential for salvation but that a Christian should nonetheless believe in order to have a healthy faith.
Getting these practical near-essentials wrong is a barrier to sound Christian faith—it is not healthy. To teach error in the near-essentials will undoubtedly result in the harm of the learner. A person can be in error on these near-essentials and still have genuine, saving faith.
A prime example of a practical near-essential is trusting that the Bible is God’s inerrant, sufficient, and authoritative Word. It is obvious from the convictional perspective: if the individuals that make up a church don’t agree on the nature of Scripture, there isn’t much chance for any functional unity.
But, what you believe about the Bible is of greater consequence that just church unity. While it is true that what a person believes about the Bible doesn’t save or condemn them, it is also true that to disregard the Bible is to disregard God Himself. Additionally, with all the changing winds of culture and the unreliability of man, if one were to deny the revealed Word of God, how would they get truth from God? Right belief about the nature of the Bible doesn’t save, but to disregard it is unhealthy at best.
Other examples of practical near-essentials are the creation/fall/redemption narrative, the attributes of God (good, perfect, sovereign, wise, loving, merciful, just, etc.), and what it means to be human and made in God’s image.
Conviction – A belief where unity is necessary for Christians to work together in the same local church.
For a new or growing believer that doesn’t go to your church or to a church within your denomination, this might be difficult. Additionally, for a new believer, we don’t want to pile obligations on them; for some things (like whether or not Scriptural baptism is by immersion), it might be best to let them figure that out over time. However, if they have the notion that Christian Perfection is true (the idea that upon belief it is possible, and even expected, that a Christian will reach a point of maturity that they will no longer sin), it would probably be wise to guide them around that. In general, be wise to help them discover God’s truth when it matters and to be patient otherwise.
Preference – A matter upon which Christians in the same congregation may disagree while maintaining local church unity.
These aren’t really matters where you should spend the bulk of your time making sure they have a Biblical worldview or share your perspective. The essential and (to a lesser degree) convictional doctrine should dominate your time and effort in the area of correction. Answer any questions that are asked of you that you can, but don’t spend a lot of your time (or even give lengthy answers) for these sorts of things.
Here is a spreadsheet to use as an exercise to think through some possible doctrinal statements and consider whether you would address them, how often you would address them, and how you would go about addressing them in a one-to-one relationship:
Here is a chart that visualizes the impact of doctrinal error. The inner (core) levels have the most impact and we should put more effort there. 
- This concept, the three categories (essential, conviction, and preference), and most of the doctrinal statements on the spreadsheet are original to Tim Raymond of Trinity Baptist Church
- The chart data and the idea are from Michael Wittmer.