It is not in the nature of an apple to exist, for it derives existence from something else. Part of its definition is not that it exists, for it simply happens to exist as an apple. If existence were a defining feature in an apple, it would be existence itself in some capacity. God however is existence by nature, for he does not derive existence from another. Existence is a defining feature of God, not an apple which derives existence from God, for he is existence itself, ultimate reality. This is why God’s nature and existence are one.
Further, God cannot be divided into parts, therefore his nature cannot be apart from his existence, lest God, the infinite being, be divisible, putting limitations on him. This is not so, therefore God’s nature and existence must be the same.