Once again, Paul Helm offers some sterling reflection on "The Classical Calvinist Concept of God." If you are unfamiliar with the history of this discussion, you may find this article tough sledding mentally. But there is one point he makes that is very important to understand.
Many men have accused Calvinists of holding to a rationalistic conception of God, especially in his works of predestination and election. But in reality, as Helm clearly shows, it is the alternative views which impose a philosophical a priori assumption upon the Bible. The Pelagian, Semi-pelagian, Arminian, Molinist, and Open Theist views all assume a form of libertarian freedom nowhere taught in the Bible. They then struggle to make this assumption fit with the biblical revelation of God, which forces them to deny or reinterpret many passages of Scripture. This is true rationalism.
1000 years ago Anselm of Canterbury stated well the proper relation between reason and revelation: "I do not seek to understand so that I may believe; but I believe so that I may understand. For I believe this also, that 'unless I believe, I shall not understand'." This epistemological attitude of the heart will keep us from reasoning away what the Bible clearly teaches.