The Kalam argument for the existence of God is a philosophical one that was popularized by William Lane Craig and is based on older arguments from Aristotle and Islamic philosopher Al-Ghazali. This deductive argument is formulated as follows:
Let’s expand on the first two premises and then the conclusion.
Experience tells us that everthing that happens, happens for a reason. To think that something could happen for absolutely not reason is unthinkable! Some scientists argue that in the realm of particle physical and quantum mechanics, things appear to happen for no reason. But the mathematics behind quantum mechanics doesn’t prove that events occur for no reason, they can only calculate the probablility of an event occuring. But at scales larger that the atoms, we know that things happen for a reason. So everything that happens happened because of some prior event and we therefore have a chain of cause and effect going backwards in time. But how far can we go back? In the section on infinity, I will present an arguments against the possibility of actual infinities occurring in nature. Note this premise includes the word “begins” which is key. Theists claim that God didn’t begin to exist and so God wasn’t caused by any prior cause. In fact most theologian claim that God created time which means that God exists in a dimension which is not constrained by time and in which God is eternal. i.e he always has and always will exist. In this sense, God is the beginning of the chain of cause and effect, the “uncause cause” of all elst that exists.
The truth of the second premise can be argued from science and philosophy. The Big Bang section provides more details about this but essentially, all the scientific evidence we have points to a beginning of the universe. This is a finite time in the past when the entire matter and energy of the universe suddenly came into existence 13.8 billion years ago from absolutely nothing. Approaching the premise from the philosophy side, it can be convincingly argued that infinity can only exist in the mind but not in physical reality. The conclusion we can draw from this is that time can not be infinite to the past and therefore time itself began to exist.
If the above two premises can be shown to hold water, then we can convincingly argue that something immensely powerful and non-physical caused the universe to come into existence. This “thing” is what theists refer to as God.