A sure foundation
"... we preach Christ crucified..." 1 Corinthians 1.23
Everybody knows René Descartes for his dictum "I think therefore I am". What they don't appreciate is the thought experiment which led to his famous statement; nor the conclusions he drew from it, which culminated in his proof of the existence of God. Nor, apparently, that the process inspired the Matrix films!
Descartes was grappling with the conundrum of reality: how can we be sure that it is not all an illusion? So he stripped away everything of which he was not absolutely certain until he was left with that single affirmation: "I think therefore I am." From there he rebuilt credible arguments for the existence of God, based on the premiss that the idea of infinite goodness must come from somewhere, and that convinces him that such a being would not behave deceptively and therefore the world must exist.
Not everyone is convinced by Descartes' line of reasoning, though it worked for him and set the course for the development of modern philosophy. Perhaps Descartes' experience can help Christians live with the frustration that not everyone shares our worldview either? The apostle Paul famously described the crucifixion of Jesus as "a stumbling block to Jews and foolishness to Gentiles" or, as the New Living Translation more vividly puts it, "... when we preach that Christ was crucified, the Jews are offended and the Gentiles say it’s all nonsense." Yet, as Paul goes on to affirm, it is "the power of God and the wisdom of God" for those who believe.
Which brings us back to where we began: how can we know for sure? And the answer depends on where we place our trust: in the power of human reason or in God. Descartes would probably argue that it is both, because God gave us reason and revelation, so that we might know him and work everything else out from there...