Is greatness measured by physical status? Not according to the Lord. As He took a child He looked upon him as being of such a mind that each disciple of the Lord must be the same. If we take John 3:3-5 and couple it with 18:3-4 we have at least three components to salvation, don’t we? Don’t tell the “faith-only” crowd. They will call that a works religion!
Jesus uses a child as a perfect illustration of the way Christians ought to be; to whom does He address His words in v. 6? Many expositors say it is mature believers (and I agree), but why should we think it is not little children as referenced in the previous verses? To begin, a child’s humility and dependence is used as an illustration for one who actually responds to the Lord’s invitation; this would refer to one who can decide on his or her own (cf. Acts 8:3). Second, the “little ones” are lost (v. 11); does this apply to a child? Third, not directly connected to the text, note how Jesus addresses the disciples (John 13:33; cf. 1 John 2:12).
The greater point of understanding in the text is not who the “little ones” are, but the necessities and consequences associated with the problems of those who cause the little ones to lose their faith (18:7). Some men and women of God will say something like this when others leave the Lord: “It’s not my problem!” Perhaps it is more than is realized.