Whatever is necessary to the essence of a thing cannot be absent from that thing. Now existence or being is necessary to the nature of a necessary being. Without existence it would not be by nature a necessary existent. It follows then that a necessary Being must necessarily exist. Here is another way to think of this argument.
Humanity can fathom the idea of a necessary, perfect Being.
Since a necessary, perfect being must exist in order to be necessary and perfect. Because we can imagine such a being, it must exist. This has long been a popular and well-regarded proof of God. But people will likely not understand it. If they do have enough basis in logic to understand it, they will probably also recognize the limitations of rationalism and therefore the limitations of this argument.