The second leg of the Kingdom Triangle is the cultivation of a tender heart, a spiritually mature inner life, a life of character and wisdom. Spiritual disciplines, while not exhausting the tools God has given us to develop these traits, represent a time-tested set of exercises unto godliness. But the Western church lost touch with spiritual disciplines, especially the Protestant branch of the church. Richard Foster's Celebration of Discipline is the single, most important book in the last century for restoring these disciplines. It's impact for Christ has been immeasurable.
Some people have rejected Foster's book for extremely harmful and superficial reasons, e.g., his suggestion that we employ visualization is therefore New Age. But this claim violates the logical fallacy of affirming the consequent: If one is a New Age person, then one uses visualization, one uses visualization (Foster), thus one is New Age. New Age folk each, drive cars, and do lots of things that believers do, but that does not make a believer a New Age practitioner. Likewise, there can be more than one reason for using, say, visualization (not to mention more than one way to practice it and more than one meaning to attach to it and more than one way to specify the goal for its employment).
At the end of the day, Foster's book has brough countless believers into a deeper walk with God, and along the way, helped many not to wear their emotional underwear too tightly.