I sometimes have doubts whether there is a God. How can I know that God really exists?
Cardinal Newman famously said that ‘ten thousand difficulties do not make one single doubt’. A ‘difficulty’ is an intellectual problem that can be resolved by reading and studying the arguments for and against the matter in question. A ‘doubt’ on the other hand refers to a psychological or spiritual state that may be brought about by an intellectual ‘difficulty’, but it can also have many other causes like lack of prayer, stress, loneliness, want of determination, trials and temptations. Both ‘difficulties’ and ‘doubts’ are common in spiritual life.
Regarding the question of whether there is a God, classical and contemporary philosophy has produced sophisticated arguments and extensive discussions on this issue. A version of the cosmological contingency argument, which starts from our experience of ourselves, is: I exist, breathe, move, perceive, live. We often take these things for granted, but none of them are obviously the case. My life could end at any moment; I am not sufficient for my own existence. There has to be something self-sufficient that created me and sustains me in being – and this we call God.
Another common argument for the existence of God is the kalam cosmological argument. The first premise of the argument is that everything that begins to exist has a cause. The universe began to exist. Hence, the universe has a cause. Some physicists and philosophers speculate about physical structures that don’t have a beginning but caused the universe. Or they simply state that the beginning of the universe is a random fact that doesn’t require any further explanation. However, the kalam argument remains very strong since both replies are on a speculative level.