Calvinists often cite Jeremiah 17:9 in support of their views. The heart is deceitful above all things, and desperately wicked, who can understand it? There is one major flaw with the Calvinist interpretation: A Calvinist's heart is also deceitful above all things, and desperately wicked, and cannot be understood.
The great thing about the Bible is that it protects us from that deceit. When my heart tells me one thing, and God's word tells me another, I must go with what God's word says. My heart tells me I'm a good person, God's word tells me I'm a sinner. Ok, so I'm a sinner, and I know it. There is nothing my deceitful heart can say to convince me I'm a good person as long as I both listen to, and apply, the very much non-deceitful word of God.
Now, obviously, I believe this verse is true. The question is, what does it mean? I believe the answer lies in a sincere study. Read the verse in context, possibly study the historical background, and if you can, nothing is wrong with studying it in the original language. Through this, I believe we can come to an adequate understanding of the verse. However, because both Calvinists and non-Calvinists alike are in view here, Calvinists must live with the knowledge that their own heart is deceitful above all things and desperately wicked, beyond even understanding. How, then, do Calvinists know that their deceitful hearts are not causing them to misinterpret this verse, and others relating to Calvinism?
Of course, the flip side is possible. Maybe the heart really is so deceitful that even the most sincere and faithful studies will lead to no greater understanding of the truth. Alternatively, maybe Calvinism is simply an imperfect understanding of a perfect truth. Undoubtedly, the Bible teaches that the heart is deceptive, man is naturally at odds with God, and our sin even blinds us to Him. But if our hearts are so deceptive that even a sincere reading of God's truth might lead us to the wrong conclusion, as it has lead so many non-Calvinists away from Calvinism, then Calvinists must live with the possibility that their own understanding of this verse, and others which convince them of Calvinism, may just be a result of their hearts doing what the heart does best: Deceiving.