“Unless you change and become like little children, you will never enter the kingdom of heaven.” –Matthew 18:3
Therapist and spiritual director Fiona Gardner draws a deeper understanding of the value of our lost innocence from the writings of Thomas Merton:
One of the characteristics of infants is that they are often seen, especially in the spiritual sense, as innocent. . . . In his essay “The Recovery of Paradise” Thomas Merton writes of the Desert Fathers and their search for “lost innocence,” which they saw as [the] emptiness and purity of heart “which had belonged to Adam and Eve in Eden. . . . They sought paradise in the recovery of that ‘unity’ which had been shattered by the ‘knowledge of good and evil.’”  . . . This unity that had been lost was, as the Desert Fathers saw it, the unity of being one with Christ. . . .
Jesus’ teaching tells us that the gift of being like a child is vital and necessary for entry to the kingdom—it is a command: “unless.” This extraordinary teaching is consistent in the three synoptic gospels but the meaning of the teaching is less clear. In fact the mystery of what it might all mean is revealed only to babies and toddlers, in other words those who are not yet able to speak: “At that time, Jesus said, ‘I thank you Father, Lord of heaven and earth, because you have hidden these things from the wise and intelligent and have revealed them to infants’” (Matthew 11:25). The message is that for us to see and to be close to God we have to relinquish the part of us that feels important and knowledgeable as a grown-up and turn in a state of not-knowing to God. . .
Moving from knowledge to innocence regained is a way of temptation and struggle; “it is a matter of wrestling with supreme difficulties and overcoming obstacles that seem, and indeed are, beyond human strength.” 
Gardner compares Jesus’ teaching in Matthew to a Zen koan, which invites listeners to hold two contradictory statements together until a new awareness arises:
If we consider Jesus’ command to the would-be adult disciples to become as small children as equivalent to a koan, then the work is to hold the lost innocence and the knowledge until the breakthrough can emerge. . . . As Merton knew from his reading on the Desert Fathers and his own spiritual practice it is not possible as an adult to regain innocence without knowledge. . .
Purity of heart is the recovery of divine likeness where the true self is lost in God. . . . This as Merton writes is “only a return to the true beginning.”  For this is where Christ is—in the beginning and in the becoming. This is the rebirth or a fresh start where Merton believed the preparation took place “for the real work of God which is revealed in the Bible: the work of the new creation, the resurrection from the dead, the resurrection of all things in Christ.” 
 Thomas Merton, “The Recovery of Paradise,” in Zen and the Birds of Appetite(New York: New Directions, 1968), 117.
 Merton, “The Recovery of Paradise,” 124.
 “The Recovery of Paradise,” 131.
 “The Recovery of Paradise,” 132.
Fiona Gardner, The Only Mind Worth Having: Thomas Merton and the Child Mind (Eugene, OR: Cascade Books, 2015), 16, 17, 24–25, 26, 27.