Luke 9:47-48

But Jesus, knowing what they were thinking in their heart, took a child and had him stand beside Him, and He told them, “Whoever welcomes this child in My name welcomes Me; and whoever welcomes Me welcomes Him who sent Me; for the one who is least among all of you he is the one who is great. 

The apostles were arguing among themselves about who would be the greatest.   Their focus was not the same as Jesus in bringing people to salvation.   Jesus came here to serve, not to be served, and to lead people out of sin.  He was calling the disciples to be leaders of men and a servant, and that would make them great.