One big stumbling block is the triune nature of God. God is one and three or God is three and then one. How can God be both? This is not normal to our understanding of the nature of being. Which is why the ontology of God is such a contentious issue. Many have their points which claim that God is unity before diversity or diversity then unity. Of course then there are those who cannot get beyond their created nature and call it all impossible. And, illogical gets mentioned somewhere along the line.
Historically the Western and Eastern Churches have differing views ontologically. The simple way to get this is that Western begins with One God and three persons and the Eastern is Three persons of the same substance unified by communion. There are three basic terms that despite having different words within reason mean the same. Substance and essence are considered interchangeable. It is considered essential to the Trinity that all three share the same essence/substance, otherwise they are not God. Person is often exchanged for hypostasis. This is because Person has shifted in its meaning over time. At first it was a theatrical term for the mask that Greek actors would wear. The Capadocian's took it and gave it a dynamic and relational meaning. The persons/hypostases within God-self share the same essence/substance with each other in a non-individual way that is in the opinion of John Zizioulas that this participation between the members of God-self is God's existence.
Think about it for a bit. Chew on this in your heart and your mind. God is not like us in his nature. God's essence is shared in a way that is dynamic and relational which is beyond any being we can see with our eyes. The word used to describe this unity in diversity is Perichoresis, it means a circle dance with three people. God is a dance. Fluid motion, never ending intwined in each other in the only perfect loving relationship. Now if thats an image of God that we can take and implement in our lives. If we use it where will we end up in our relationships with others?
I often scour the internet looking for images of Perichoresis and the Trinity, most are disappointing. This one comes from www.flickr.com/photos/andreacyclop/ and is wonderful in its movement. There is something about abstract expressionism that grasps the Trinity in way more representative art cannot. This has to do with the fact that Abstract art does not attempt to capture a representation of the scene, person or landscape, but, to merely depict it. God is beyond us. In the nature of God-self there is a dynamic relational existence that we can only depict in word images, metaphor and analogy. Theologians try and represent Gods ontology but often end up grasping at words like person, essence, and perichoresis because God can only be depicted by us at this time.
Augustine talks of seeing God. Seeing for Augustine is knowledge imparted to us by scripture and the creeds (in Augustine's case the Nicene Creed). For him the pure in heart will see God (Matt 5:8) now because their faith has believed and they now 'see" God. But God is invisible and Augustine asserts this to be true but that one day when all is under the rule of Christ it will be given to the Father (Phil 2:5-7). It is then that the pure in heart who believe by faith will see God-self. I am looking forward to that day where I will see with my eyes something that I can only grasp at now.