Dallas Willard, who, before his death 2 years ago, was a professor of philosophy at USC and a key thinker and writer about Christian discipleship, has a simple diagram that explains well the process by which God brings about change in our hearts as Christians.
Here is the diagram (from page 347 of his landmark book The Divine Conspiracy), which Willard calls “the Golden Triangle of spiritual growth.” The triangle represents the correlation of factors which lead to transformation into Christlikeness. The center of the triangle has to do with our thinking, showing that, as Romans 12:1-2 states, we are transformed by the renewing of our minds—that the changing of our thinking (about God and about self especially) is central to the whole process.
Then at the apex of the triangle is the action of the Holy Spirit, demonstrating that the work of the Spirit is primary and vital to transformation—this is not merely a self-help project. Ordinary events and trials of life, coupled with spiritual disciplines (or heart-training exercises, as I like to call them) indicate that where the transformation is actually carried out is in our real, everyday life where we interact with God and people. The role of what is imposed upon us (ordinary events and trials) goes hand in hand with our intentional efforts to open our hearts to God (disciplines).
So God moves, through His Spirit, to awaken our minds to His truth–to correct our perception of Him, to bring awareness of the depth of our sin and need of a Savior, to point out ungodly beliefs and correct them with the truth. God changes our thinking and renews our mind. But He doesn’t just stop there—we are not changed simply because we learn a new truth or feel convicted about a particular command. Changing our thinking is central, but it does not produce transformation on its own. Rather, what is changed in the thinking must then be lived out in the everyday events of life and proven in the testing grounds of ordinary struggles and suffering and relationships. We cannot directly cause the transformation to happen, but we can choose to put ourselves in a posture of openness and receptivity to the Spirit’s work—and that is what heart-training exercises accomplish, disciplining ourselves for the purpose of Godliness, as I Timothy 4:7 states.
UPDATE: I preached a sermon at Lifesong Community Church this past Sunday (May 24, 2015), in which I gave a couple illustrations of this “golden triangle” in everyday life. You can find the sermon here.