The answer is, the children, adult children who have preserved purity, spontaneity, and a kind heart in the path of life in spite of all trials and sorrows.
Once the apostles asked the Savior, who holds the highest position in the hierarchy of the Heavenly Kingdom? People, both in the past and present, tend to think about heavenly things in earthly images and categories. The kingdom presupposes a ruler, high officials, an army, a state bureaucracy, and ordinary citizens. On earth, we consider “lucky” those who manage to “hit the jackpot” or those who are born in the “right” family and climb the social ladder to a pleasant height. In apostolic times, common understanding of happiness was no different. It has always been great to be a king (even if of very local significance) or Roman procurator; it has always been good to have the honor and respect of a Pharisee or make a prosperous living being a Republican. The Savior promised the apostles a new, eternal Kingdom in which they could find happiness. It is reasonable that the disciples ask the Teacher about who of them would enjoy greater benefits in the Abodes of Heaven?
The Savior called a child to come to the circle where He was sitting with the apostles. Taking the child in his arms, Christ placed him among the disciples. Jesus’ actions probably caused some confusion among the apostles: how does the child have the highest position in the Kingdom of God? Of course, the Savior spoke figuratively, but He always gave the best example intelligible to human understanding. What is the difference between a small child and an adult? It is the child’s innocence. “Where there is simplicity, there are a hundred Angels, but where there is cleverness, there are none,” the Venerable Ambrose of Optina used to say. Without doubt, by “being like children” the Lord meant their pure inner world, not yet touched by the worldly taint. Look at children.
Children follow their father, love their mother, do not wish harm to their neighbors, do not care about wealth, do not feel proud, do not harbor hatred, do not lie, and believe absolutely everything. They are not persistent in anger, quickly forget offences, and have no weakness for the opposite sex. And most importantly, this innocence, simplicity, and purity of spirit in children are absolutely natural! Yes, we have to be honest and admit that even children are not perfect and often are not “equal to angels” at all. Inheriting the character of their parents, children absorb, like a sponge, everything that happens around them. Some behavioral patterns borrowed from adults are far from childish and look creepy and unattractive when used by a child. But it is because of the adults who, willingly or unwillingly, bring up “these little ones” through their example. Christ does actually warn us that grief will befall the person who “offends” (i.e. sets a bad example to) a child. By the way, that also applies to relations between adults where spiritual depravity, which is becoming more and more common in society, captures the hearts of its victims. It is twice as hard for Christians: they do not want to adapt to a spiritually hostile environment, because they understand its nature, but their weaknesses do not always allow them to resist what is happening in front of their eyes which causes them suffering under the pressure of obscenity.
I must admit: we are seriously stuck in this world. Our souls have undergone serious changes chasing worn-out values. They have become rough under the scale of sins. But aren’t we searching for a way to the Kingdom of Heaven? If we still are, then from under the thick layer of the worldly values we need to dig out the genuine ones. We need to heal by repentance the wounds caused by sin and revive our mortified hearts with the Body and Blood of the Savior. Above all, we need to… become children, grown-up children with a pure heart, good intentions, and deedful love.
Translated by The Catalogue of Good Deeds
Source: https://pravlife.org/ru/content/kto-bolshiy-v-carstve-nebesnom