The first Ontological Argument comes from Anslem of Canterbury in the 11th century.
Dinesh D’Souza who wrote What’s So Great About Christianity explains it this way, “Anselm defines God as ‘that than which no greater can be thought.’ Presumably, this is a reasonable and widely accepted definition. Even an atheist should have no problem with it. We all understand the idea of God to correspond to a supreme being that stretches—even transcends—the limits of our imagination. Anselm proceeds to say that as we acknowledge and understand the definition, we must have some idea of God in our mind. He doesn’t mean a pictorial representation. He simply means that our minds comprehend as a logical possibility the idea of God as ‘that than which no greater can be thought.’”
Ontological arguments can use reasoning and priori for evidence. What is a priori? A priori is knowledge that is independent of experience. For example, 1+1=2. You can calculate this, (at least I hope so), without having to arrange two groups of single objects, and count the total. Another common example of a priori is the statement, “All bachelors are unmarried”. You know and understand that bachelors by definition are unmarried, so all bachelors are unmarried. Another way to help you understand a priori is to consider something that is not one. A statement that is not a priori would be, “I have a cookie in my pocket.” You actually would not know if I had a cookie in my pocket unless you had the experience of placing it there, seeing it placed there, or feeling it there.
Another line of reasoning that R.C. Sproul used in, Defending Your Faith An Introduction to Apologetics, was to consider how the device, (computer, iphone, ipad) you are using to read this text came to be. In other words, how did it come to exist? There are ultimately four possible explanations to its existence. It is an illusion, it has always existed, it was the cause of its own creation, or it was created. Each of the possible answers can be researched or discussed at length, and some philosophers and scientists have done so, but as Christians, we know the answer is creation.
I don’t think the current cultural revolt against God is primarily based on an intellectual debate. Until a couple of years ago I never gave much thought, (like many Christians), to the various arguments regarding the existence of God. Like a child, I accepted things I was taught as a child, despite what the world was teaching me. I believe the cultural revolt against God is more moral in nature; people don’t want to be held accountable for their behavior. In 2012, I received a Bible verse at church, “Guide me in Your truth and faithfulness and teach me…” After that, God moved in my life and placed on my heart a deep desire to know Him more, by His beautiful creation, His brilliant design, His perfect character, His omniscient knowledge, and historical evidence; all left for us to discover and share with others so they will be guided and taught of His truth and faithfulness.
Psalm 25:5 Guide me in your truth and teach me,
for you are God my Savior, and my hope is in you all day long.