In modern times many would consider Dallas Willard to be the father of modern understanding of spiritual formation, the following explanation is an extract of his fuller article found here
Spiritual formation could and should be the process by which those who are Jesus’ apprentices or disciples come easily to “do all things whatsoever I have commanded you.” What I call “the great omission from the great commission” is the fact that Christians generally don’t have a plan for teaching people do everything that he commanded. We don’t as a rule even have a plan for learning this ourselves, and perhaps assume it is simply impossible. And that explains the yawning abyss today between being Christian and being a disciple. We have a form of religion that has accepted non-obedience to Christ, and the hunger for spirituality and spiritual formation in our day is a direct consequence of that.
Spiritual formation in the tradition of Jesus Christ is the process of transformation of the inmost dimension of the human being, the heart, which is the same as the spirit or will. It is being formed (really, transformed) in such a way that its natural expression comes to be the deeds of Christ done in the power of Christ.
The progression of spiritual formation is outlined in various passages of the New Testament. It is most fully spelled out in II Peter 1: “Now since you have become partakers of the divine nature,” the writer says, “applying all diligence, in your faith supply moral excellence, and in your moral excellence, knowledge; and in your knowledge, self-control, and in your self-control, perseverance, and in your perseverance, godliness; and in your godliness, brotherly kindness, and in your brotherly kindness, love.” (vv. 4-7)
These New Testament progressions always conclude with agape. Agape is the center, the linchpin, of it all. Colossians 3 has a wonderful progression that concludes, “And beyond all these things put on love, which is the perfect bond of unity.” (v. 14) Romans 5 concludes its progression with the words, “because the love of God has been poured out within our hearts through the Holy Spirit who was given to us.” (v. 5)
If you examine these and related passages you will see that they include a passive element and an active element. And making the distinction between passive and active, and seeing how they come together, poses–especially for the evangelical understanding–the greatest difficulty in the area of spiritual formation.
We know, as Jesus says, “Without me you can do nothing.” (John 15:5) It is the initiative of God and the presence of God without which all of our efforts are in vain–whether it is in justification or sanctification or in the realm of the exercise of power, all our efforts will be in vain if God does not act. But we had better believe that the back side of that verse reads: “If you do nothing it will be without me.” And this is the part we have the hardest time hearing.
Spiritual formation talk has emerged within evangelical circles because of a pervasive felt need–felt on the part of many people within the laity as well as within the clergy–for “something more” than the group and individual activities that have been recognized and encouraged in conservative religious circles in recent decades. I think that during the period since WW II, especially, we came to accept the marginalization of discipleship to Jesus. We came to see it as something of an option that we might choose to exercise should we wish. But if we would just like to believe the truth and receive the ministry of the word, and get on with our life without discipleship, that’s okay too. And as a result we have now come to the place where we can be a Christian forever without becoming a disciple.
So discipleship was marginalized to something that was a special function. In the path of serious spiritual formation there is indeed (as there always has been) a real possibility of meeting the need for transformation. There is a real possibility of looking at I Corinthians 13, for example, and being able to see that the love that is portrayed there can actually come to occupy the human heart. People can really be like that–“Bears all things, believes all things, hopes all things, endures all things.” People can be like that, not because they do such things, but because agape love has occupied them effectively as a result of their having learned how to receive it into the deepest part of their being.
Now I think we are at a crucial point in time, and that the very great promise which currently steps forward in the “buzz” about spiritual formation has three possible outcomes, two of them disappointing ones.
One is that spiritual formation will be lost in the sea of humanistic or “new age” spirituality. The hunger of the human heart that is unfed by what is authentic will go for what is inauthentic. If human beings need something vital badly enough, they may even destroy themselves trying to get it.
Secularism itself [is also presented as ] a ‘spirituality’. Even a mere “culture” has a spirituality to it now, the claim is that secular people have a ‘spirituality’. Spirituality is taken to be simply one dimension of the human being. From the scriptural teachings we know that that is supposed to be right–human beings are, as such, supposed to have a spirituality, and in a sense they do. They remain spiritual beings, with all that implies. But on their own they’re dead spiritually. They’re cut off from the source of spiritual life.
Another possibility is perhaps more dangerous, it is that spiritual formation has simply become a new label for old activities–for what we are already doing: worship, hearing the word, community, quiet time, plus a new twist or two such as spiritual direction and so on. Now all of these things are very important. But if spiritual formation merely becomes a new label for things we are already doing, it will leave us right where we are. And the issues of deep inner transformation will remain untouched. And I say with trepidation that there is a real danger of spirituality becoming a field of mere “expertise,” of academic competence, focused upon “religious activities.”
I think that one of the greatest dangers for the cause of Christ today is that we Evangelicals will not understand our need for genuine repentance: repentance, not about what we aren’t, but about what we are. Our problem is not caused merely by the fact that we don’t do certain things, like love our neighbor as ourselves and so on. It’s the very things that we teach and practice about the spiritual life that leave us in the position of not doing the things we should.
Another possibility is that “spiritual formation” could become a term for those processes through which people are inwardly transformed in such a way that the personality and deeds of Jesus Christ naturally flow out from them when and wherever they are. In other words, it can be understood as the process by which true Christlikeness is established in the very depths of our being. Thus multitudes of men and women could be brought forth from generation to generation to be, unapologetically, Christ’s redemptive community: the true “city set on a hill,” of which Jesus spoke, established in the midst of the earth now, as it shall be for eternity in the midst of the cosmos. (Eph. 3:10; Rev. 22:5) We could become a true “society of Jesus.” We could be the life-transforming salt and light in a darkened world which God has always intended his covenant people to be.