Explain how Descartes developed Anselm's argument that God's existence is necessary

Anselm's second argument concludes that God has to exist and cannot fail to exist, in philosophical terms this is called necessary existence. Anything which has to exist and cannot fail to exist is said by philosophers exist by necessity. Most things that exist depend on something else for their existence, for example, your home only exists in reality because some built it, you cannot say your home had to exist, because it was up to the builder to decide whether to build it. This type of existence is identified as contingent existence. Contingent existence refers to something which depends on referring to something else to fully explain why it exists.

In reference to the question, Descarte developed Anselm's argument, that God's existence is necessary. Descarte claim that God has placed in every person the idea of God. Descartes likened this idea to a trademark or a stamp that identifies an object as belonging to a craftsman. Some things cannot be doubts such as God's existence. Demonstrating God's existence is not about proving the idea of God true or that it is more than an intuition. As this point was derived from Anselm's necessary existence argument, as descarte suggested in his ontological argument is that the existence of God cannot be doubted, just as the truth of mathematics cannot be.  

Descarte uses the triangle as an example, the nature of a triangle is that it has three sides and three interior angles, adding up to 180 degrees, the nature of the triangle is called immutable by Descarte; meaning incapable of change or being different. The crucial point is that, like a triangle God also has an immutable nature. According to Descarte, part of God's nature is that God exists, thus making immutable; it tells us something about God. Part of God's essence is therefore existence. Descarte argues that God existing is as fundamental to the nature of God as the interior angles of the triangle. For Descarte, the essence of God is that God exists and existence is a predicate of God. As Anselms put forth the definition of predicate, in philosophical terms, to indicate an intrinsic property or quality of something, therefore the predicate of God is His existence.

 Descarte went on to further develop Anselm's argument that God's existence is necessary by using three simultaneous arguments, firstly, God is a supremely perfect being, secondly, a property perfect is existence, therefore God existences. Existence, so far as Descarte is concerned, is a perfection of God in the same way as a mountain and a valley going together. You cannot talk about God unless God exists because God is perfect and part of perfection is existence.