This is a topic that in many ways I know very little about. I can claim no great spiritual heights nor theosis. Nevertheless, it is becoming increasing apparent to me what it means to know God and how this is achieved.
Knowing God is not an intellectual exercise nor even intellectual knowledge as such, although this is not excluded from participating in knowing God. This type of knowledge only grasps images and concepts about God but it cannot know God; this is something different. God is not a concept or image but a reality that it not containable in one’s mind. However, we do get to know God in a manner in what the Fathers call the nous. This is said but most to reside in the heart rather than the head. Thus, it is not a reasoning, intellectual knowledge such as mathematics but rather it is an experiential, intuative knowledge that is impossible to describe or compare to normal human experiences but has a little to do with wisdom.
God is known in the heart in the way that we participate in His life and this life becomes ours. We learn this initially through obedience to the commandments and then as we grow in virtue and purity through the participation in the Spirit. Eventually, we love as God loves, we think as God thinks and so thus we know Him because we realise that the way we live is how He lives. We share the same motivation, the same understanding and the same will; we want what He wants. This knowledge is in a sense active but it transcends physical activity and acts in a spiritual manner with far greater reach than of which physical human actions are capable. This knowledge is what is said to participate in God’s energies or operations. These are not something that happens to us from outside but come from within and become our own, yet remaining God’s and coming from Him. This is theosis or union with Him.
The above knowledge though is not that of the essence. We cannot attain to this that is because you have to be something to experience or know its essence. We can neither know the essence of a tree nor a dog because we are neither trees nor dogs. Thus, even though we are united to God in His life we will still know that He is somehow other to us and His essence is not ours. He is God and we are man. This cannot be changed. Nevertheless, by living His life we become gods because all of His life is ours, if we are willing to accept it as it really is.