The heart is more deceitful than all else and is desperately sick; who can understand it? (Jer 17:9)Starting from the basis that God's Word is true, we will have to, then, concede that the heart of natural man is deceitful. Knowing that might help. "Hey, I have a deceitful heart. I need to watch out for that." On the other hand, it should pretty easily be recognized that now we have a problem. If we admit that we have deceitful hearts, how can we know the truth? And if we figure out the truth, how do we know we're not being deceived ... by our deceitful hearts?
Scripture speaks a lot about truth. Jesus called Himself the Truth (John 14:6). The psalmist declared, "The sum of Your word is truth." (Psa 119:160) Jesus prayed, "Sanctify them in the truth; Your word is truth." (John 17:17) Falsehood is found in adulterating the word of truth (2 Cor 4:2), so we are to accurately handle the word of truth (2 Tim 2:15). God's word is truth (2 Tim 3:16-17), so we have an external standard, outside of our own hearts, with which to determine what is true.
Well, that's certainly handy, but ... we still have a problem. While admitting up front that God's word is truth (and when they do not, you can immediately dismiss them, having only their own deceitful hearts as their standard), we are still left to our own wits, so to speak, to interpret God's word. The question, then, is not whether or not His word is reliable; the question is whether our understanding is reliable. Back to that pesky "deceitful heart" problem. Because, as we all know, it is quite common for people to cite the Bible for their truth claims in opposition to others who do the same. Who is deceived?
The problem seems sticky at the front, but I think with a little effort we can ease most of it. If the Bible is true, then it's pretty easy, in fact. First, is your understanding of the text consistent with the text? Considering the language, the content, the writing style (historical, doctrinal, hyperbole, poetry, etc.) -- considering it all, is your understanding consistent with the text? Next, is it consistent with the context? I've heard it said, "Never read just one verse." Does your understanding of a text take into account the context of that text and is your understanding consistent with that context? Third and obviously, is your understanding of the text and the context consistent with the rest of Scripture? If we assume up front that God breathed this book we call the Bible, it would have been done in truth, so nothing in it should contradict anything in it. Is your understanding consistent with that principle? In short, are you letting Scripture interpret Scripture? One more step. Jesus said, "When He, the Spirit of truth, comes, He will guide you into all the truth." (John 16:13) Now, we'll assume that the Holy Spirit came as Jesus promised and Scripture claims and we will assume that He has done His job. So, how does your understanding of the text, the context, and the Scriptures line up with the history of Christendom? Is your understanding novel? Does it contradict all prior understanding? Have you come up with a new and creative way to understand God's Word? Then you can be pretty sure your deceitful heart is doing its devilish work.
It is a valid concern. Unbelievers, Scripture says, are futile in their thinking (Rom 1:21; Eph 4:17). Believers are renewing their minds (Rom 12:2). As we grow, we are constantly washing out the old and replacing it with ... truth. If your understanding of Scripture is 1) in opposition to the text, the context, or the rest of Scripture and/or 2) in opposition to previous understanding, it's likely you've slipped a cog -- your deceitful heart has tricked you. John wrote that deceitful spirits have come out (1 John 4:1). "They are from the world," he wrote, "therefore they speak as from the world and the world listens to them." (1 John 4:5) If the world is consistently happy with your understanding of Scripture, chances are it's not the proper understanding of Scripture. Despite the concerns of the deceitful heart, we can eliminate most of those concerns if we allow Scripture to speak for itself, contradicting our world's views while consistently holding to God's truth. The tricky and innovative ways some are using to interpret Scripture in the face of Scripture and history don't bode well for them.