In a discussion on Calvinism, I stated, "I would advise any reading this thread not to hinge their belief of Calvinism on this verse or discussion alone. ( Ephesians 4:14 ) This is a very deep topic. I would also counsel against taking this sort of scriptural meat on when you know you have other sins or deficiencies in your service to Christ that need resolving."
I said this as a warning to others. But it was also directed at a particular person I suspected had sin issues he was avoiding. My advice seemed wise, but do you think it was scriptural?
I have a hunch (gut, no scripture) that if you know you are in sin or know your service to Christ is somehow deficient, and you refuse to act to correct that, but instead pursue investigations of questionable matters and false doctrines; that this is a problem. And further, that in such a state you may be unable to discern the truth from error. Do you think this to be true from Scripture?
I'd love any Scripture you can bring to bear on this subject. I think II Peter 3:15-16 may come into play here, but it isn't very specific ('some things').
There are several ideas here that need to be addressed.
Are Some Topics Harder than Others?
As you noted, Peter stated, "and consider that the longsuffering of our Lord is salvation--as also our beloved brother Paul, according to the wisdom given to him, has written to you, as also in all his epistles, speaking in them of these things, in which are some things hard to understand, which untaught and unstable people twist to their own destruction, as they do also the rest of the Scriptures" (II Peter 3:15-16). While the precise topics are not listed, what we need to take with us from this verse is the cause. People untaught in the Scriptures tend to misinterpret what the Scriptures state. The reason is simple: most of the Scriptures do not stand alone. A person lacking knowledge of the Bible as a whole can easily latch onto an idea taken out of context and run the wrong direction with it. In addition, unstable people will also distort the Scriptures. These are people who are uncertain about their stand in the Scriptures. For example, a person who wants to please everyone will end up distorting the Scriptures in a vain attempt of making what God said appealing to everyone.
Keep in mind Paul's warning, "Now the purpose of the commandment is love from a pure heart, from a good conscience, and from sincere faith, from which some, having strayed, have turned aside to idle talk, desiring to be teachers of the law, understanding neither what they say nor the things which they affirm. But we know that the law is good if one uses it lawfully" (I Timothy 1:5-8). Some people are pulled astray by pride. I've seen it happen many times, especially among younger Christians. There is a desire to leave one's mark on the world. A desire so strong, that a person quickly dives into topics for which they have no foundation and they end up teaching false doctrine because they don't understand what they are talking about.
"For though by this time you ought to be teachers, you need someone to teach you again the first principles of the oracles of God; and you have come to need milk and not solid food. For everyone who partakes only of milk is unskilled in the word of righteousness, for he is a babe. But solid food belongs to those who are of full age, that is, those who by reason of use have their senses exercised to discern both good and evil. Therefore, leaving the discussion of the elementary principles of Christ, let us go on to perfection, not laying again the foundation of repentance from dead works and of faith toward God, of the doctrine of baptisms, of laying on of hands, of resurrection of the dead, and of eternal judgment" (Hebrews 5:12-6:2). Here we find a list of topics that the writer of Hebrews calls foundational or "milk." It is interesting that these subjects are the ones a good number of people focus their efforts upon. It is not that these are unimportant subjects, but they are ones that do not need to be readdressed over and over again. Notice as well that it takes practice to distinguish both good and evil.
Can Personal Sins Blind You to the Truth?
Personal sins can interfere with your growth as a Christian. "Therefore, laying aside all malice, all deceit, hypocrisy, envy, and all evil speaking, as newborn babes, desire the pure milk of the word, that you may grow thereby, if indeed you have tasted that the Lord is gracious" (I Peter 2:1-3).
Peter also has a list of virtues Christians ought to have: faith, virtue, knowledge, self-control, perseverance, godliness, brotherly kindness, and love. He goes on to say, "For if these things are yours and abound, you will be neither barren nor unfruitful in the knowledge of our Lord Jesus Christ. For he who lacks these things is shortsighted, even to blindness, and has forgotten that he was cleansed from his old sins" (II Peter 1:8-9).
So can a lack of applying what a person learns to his life. That is Peter's point in II Peter 1:10. James also discusses this, "But be doers of the word, and not hearers only, deceiving yourselves. For if anyone is a hearer of the word and not a doer, he is like a man observing his natural face in a mirror; for he observes himself, goes away, and immediately forgets what kind of man he was. But he who looks into the perfect law of liberty and continues in it, and is not a forgetful hearer but a doer of the work, this one will be blessed in what he does" (James 1:22-25). This is why Paul warned about people who were always learning, but never applying things to their lives (II Timothy 3:6-7).
If you cannot understand the Bible enough to make changes in your life, obviously you don't have enough understanding to teach others about it. "However, we speak wisdom among those who are mature, yet not the wisdom of this age, nor of the rulers of this age, who are coming to nothing. But we speak the wisdom of God in a mystery, the hidden wisdom which God ordained before the ages for our glory, which none of the rulers of this age knew; for had they known, they would not have crucified the Lord of glory" (I Corinthians 2:6-8). Later Paul states, "These things we also speak, not in words which man's wisdom teaches but which the Holy Spirit teaches, comparing spiritual things with spiritual. But the natural man does not receive the things of the Spirit of God, for they are foolishness to him; nor can he know them, because they are spiritually discerned" (I Corinthians 2:13-14).
Is this your acquaintance's particular problem? That I cannot answer. The potential is definitely there.