The trouble today is that believers are not involved in being transformed in the renewing of the mind. We are seeing more and more people involved in what we call Christian fellowship for social reasons. They might experience social fulfilment, but their lives are not changed. Leaders in the church have been baffled for years about how to help people with their problems. The problem is the mind is still corrupt. It needs to be transformed and changed, which is why God has given us His Word and given us His Spirit.
"The ministry to others is written on our hearts," Paul says, "and the hearts of others." It is interesting. We are not salespeople, going out reaching people for Christ. We are living letters. If I realize that I am a letter, I am a book. Everywhere that God takes me, people are coming to know me, and people are reading me. My life has an expression to it, a message, a story. And Paul says, You are our letter, written in our hearts." And he goes on to say that cannot happen without the Spirit of Christ. Because it is not with ink and paper, it is with the Spirit of Christ that they are being changed.
This is the answer to everything about a person not being able to understand, or know, or realize, or receive the spiritual things of God. I probably have heard this more than anything over the years - people expressing wonderment that people "do not understand." That, "People just don't get it." They can't! There is a veil over their heart so that they cannot hear or understand. That veil is there until Christ takes it away. And the more they hear, and the more they reject, the more their heart becomes calloused to where they cannot be sensitive to the things of God any longer. The veil is only taken away in Christ. Christ takes that veil away for people to be able to understand.