The first, and best-known, ontological argument was proposed by St. Anselm of Canterbury in the 11th. century A.D. In his Proslogion, St. Anselm claims to derive the existence of God from the concept of a being than which no greater can be conceived. St. Anselm reasoned that, if such a being fails to exist, then a greater being — namely, a being than which no greater can be conceived, and which exists - can be conceived. But this would be absurd: nothing can be greater than a being than which no greater can be conceived. So a being than which no greater can be conceived - i.e., God — exists.
The single most important argument in my opinion is the one that appears in his Proslogion 2. The argument go as follows.1) It is possible to say that God is a being than which none greater can be imagined (ie, the greatest possible being that can be imagined) meaning that 2) God exists as an idea in the mind. 3) A being that exists as an idea in the mind and in reality is greater than a being that exists only as an idea in the mind. 4) Thus, if God exists only as an idea in the mind, then we can imagine something that is greater than God (ie, a greatest possible being that does exist). 5) But we cannot imagine something that is greater than God (for it is a contradiction to suppose that we can imagine a being greater than the greatest possible being that can be imagined.) 6)Therefore, God exists.
In my opinion there are two parts to this argument. The first premise “.It is possible to say that God is a being than which none greater can be imagined” , is that we have a coherent idea of a being that instantiates all of the perfections. Otherwise put, premise 2 asserts that we have a coherent idea of a being that instantiates every property that makes a being greater, other things being equal, than it would have been without that property (such properties are also known as "great-making" properties). Premise 3 asserts that existence is a perfection or great-making property.
Accordingly, the very concept of a being that has all perfections implies that it exists. Suppose “G” is a being containing all the perfections and suppose “G” doesn't exist (in reality). Since Premise 3 asserts that existence is a perfection, it follows that “G” lacks perfection. But this contradicts the idea that “G” is a being that contains perfections. Thus, according to this reasoning, “G” exists