Only those who are striving to "walk in the Spirit" can understand spiritual things (14-15). I have observed in my years of being a Pastor this important but profound truth. Until I am awakened to the presence and the redeeming grace of God I cannot perceive nor can I understand the life in faith. When I first traveled to Africa (in 2006) I was told many stories, I saw photos, I saw videos, I heard first person accounts of what life in Zimbabwe might be like. I filed all that information away and believed myself prepared for the experience. I was not prepared for the experience. When I was immersed in the Zimbabwe world I had a change of heart and a change of world view that no amount of story telling or photo showing could have accomplished. With this change of heart and world view I understood (or, perhaps put more humbly) I began to understand and a whole new world opened up to me. I could not have had this change of heart without the experience of being there and meeting the people.
The same is true of the life in the Spirit. I can tell people about it, I can show examples and introduce others who are on the same journey but none of that compares to the reality or the spiritual life and that reality has to be personally experienced -- it cannot be taught it must be caught. C.S. Lewis once suggested that the best way to experience this, from a doubter's perspective, was to take a month and live one's life as if the Christian message were true and see what happens. I suspect that when I spend that month reading the Bible, taking time to pray, engaging in mission and ministry and seeking worship, the Holy Spirit will meet me in each and every one of those points of contact and I will discover that my heart and life has been renewed by the presence and the power of God through Jesus.