Eph1:18 I pray that the eyes of your heart may be enlightened in order that you may know the hope to which he has called you, the riches of his glorious inheritance in his holy people,NIV
When the scripture talks about the heart, it is either referring to the mind or the human spirit. When capital letter "H" begins the word 'heart', it is talking about the human spirit but if it is written the other way, it is referring to the mind. In context of the opening text, 'heart' refers to the mind not the organ pumping blood in your chest. The mind is the seat of intellect and understanding. Paul prayed for the Ephesian believers that the eyes of their heart be flooded with light.
The opening of the eyes of the heart refers to the ability to grasp and understand things. Paul prayed that the figurative eyes of the heart be enlightened. There are those who have heard powerful sermons and stayed under sound teaching for years yet they lack understanding about what they've been taught. There are people who are ever learning but do not increase in understanding. When Philip met the Ethiopian Eunuch, the question he asked him was, "Do you understand what you are reading?” (Acts8:30). This is because understanding is crucial. God spoke through prophet Isaiah saying, "He said, “Go and tell this people: “ ‘Be ever hearing, but never understanding; be ever seeing, but never perceiving.’(Isa6:9).
It is spiritually fatal to be without understanding. When you understand the word, you'll grow in the things of the Spirit. Long ago, I discovered that accuracy in spiritual things is a product of understanding and insight. The Pharisees, who were the custodians of the law had so much problems with Jesus because they lacked understanding about the prophecies written about the messiah aforetime. It is possible to be a "theologian" and still lack understanding about God. Acquiring lettered knowledge( about God) becomes a waste if understanding is lacking. As you study the scriptures and learn about God, pray this Pauline prayer.
Father, open the eyes of my heart so that I may know the hope to which he has called me and the riches of his glorious inheritance in Jesus name