This is a tough one, depending on what you mean by "know." The best I can offer you is the evidence presented. First, there is nature itself. God's fingerprints are all over it. Paul said as much in Romans 1:20. The intricacy, the complexity, and the beauty all point to an intelligent source. Second, God has made himself known in history. In fact, the testimony of the Bible is about a God who interacts with particular people in particular places at particular times (for example, Isaiah 6:1). (The dating of events and archaeological discoveries reinforce this.) God even came among us through Jesus of Nazareth (see Luke 3:1-2 for the details of timing). We continue to meet people to this day whose changed lives point to a wonderful heavenly Father. Third, we have the witness of the Bible. God inspired the authors with the Word so that His voice could continue to be heard throughout the ages. In the Bible we meet God and come to know His will and His way. Finally, there is the continuing work of the Holy Spirit who addresses the heart with the love of God (see John 16:13). Do we know God exists? I do-we've talked.