Scholars agree that Luke used Mark's gospel as one of his sources (cf. Luke 1:1-4). When we read Mark and Luke side-by-side, we occasionally observe changes that Luke makes to Mark's account. Consider, for example, the slight modifications Luke makes to Mark 10:13-23.
Mark 10:13-23 (NASB)
Luke 18:15-24 (NASB)
13) And they were bringing children to Him so that He might touch them; and the disciples rebuked them.
15) And they were bringing even their babies to Him so that He might touch them, but when the disciples saw it, they began rebuking them.
14) But when Jesus saw this, He was indignant and said to them, "Permit the children to come to Me; do not hinder them; for the kingdom of God belongs to such as these.
16) But Jesus called for them, saying, "Permit the children to come to Me, and do not hinder them, for the kingdom of God belongs to such as these.
15) Truly I say to you, whoever does not receive the kingdom of God like a child shall not enter it at all."
17) Truly I say to you, whoever does not receive the kingdom of God like a child shall not enter it at all."
16) And He took them in His arms and began blessing them, laying His hands upon them.
17) And as He was setting out on a journey, a man ran up to Him and knelt before Him, and began asking Him, "Good Teacher, what shall I do to inherit eternal life?"
18) And a certain ruler questioned Him, saying, "Good Teacher, what shall I do to inherit eternal life?"
23) And Jesus, looking around, said to His disciples, "How hard it will be for those who are wealthy to enter the kingdom of God!"
24) And Jesus looked at him and said, "How hard it is for those who are wealthy to enter the kingdom of God"!
Note that while Mark uses the word "children" (παιδίον), Luke uses the more specific word "babies" (βρέφος). Note also that while Mark describes the rich man as simply a "man," Luke describes him as a "ruler."
Now why does Luke make these changes?
Note first the connection between these two episodes. In his encounter with the children, Jesus declares, "Whoever does not receive the kingdom of God like a child shall not enter it at all" (Mark 10:15). In his encounter with the rich young ruler, on the other hand, Jesus declares, "It is easier for a camel to go through the eye of a needle than for a rich man to enter the kingdom of God" (Mark 10:25). Jesus thus uses both encounters to make a point about the type of person who is able to enter the kingdom of God. The children, who are initially dismissed, turn out to be the standard for entering the kingdom, while the man, who is evidently a person of some wealth and importance, is ultimately unable to enter.
In his redaction of this story, Luke is apparently highlighting this contrast. As noted above, Luke changes "children" to "babies" and "man" to "ruler." These extra details serve to lower the status of the children and raise the status of the man, thus sharpening the contrast between the two. Furthermore, by removing the material in Mark 10:16-17a, Luke brings these two episodes closer together. By omitting Mark 10:16, the encounter with the rich young ruler follows immediately upon the statement concerning entrance into the kingdom (Mark 10:15), which, as noted above, is what links these two episodes together. Likewise, by omitting Mark 10:17a, Luke eliminates the temporal and spatial distance between these two encounters. In Luke's version, the rich young man appears to approach Jesus immediately after the encounter with the children. By bringing these two stories closer together, Luke makes it easier for his readers to notice the contrast between them.
Finally, note the episode that Luke has placed immediately prior to the encounter with the children: the parable of the Pharisee and the sinner (Luke 18:9-14). This parable, which is unique to Luke's gospel, ends with these words from Jesus: "Everyone who exalts himself shall be humbled, but he who humbles himself shall be exalted" (18:14). It is precisely this "principle of status transposition" (Joel B. Green) that is illustrated by the subsequent stories of the children and the rich young ruler.
In conclusion, the subtle differences between Mark and Luke may at times reveal Luke's particular theological interests and emphases. Attending to these differences may thus enhance our understanding of Luke's gospel.