Mark 10:21

And Jesus looking upon him loved him, and said unto him, One thing thou lackest: go, sell whatever thou hast, and give to the poor, and thou shalt have treasure in heaven: and come, follow me.

The one thing lacking in the young man was his renunciation of all trust in worldly things and following Christ.

Go sell ... and give to the poor ... This may not be viewed as an invariable requirement of all who would be saved; why then did Jesus lay down such a requirement here? It has been suggested that the Lord saw the cancer of greed that was eating out his heart and prescribed the drastic surgery of getting rid of his wealth. A more likely explanation is that the Lord was calling him to be an apostle, all of the apostles having been required to leave all that they had and follow Jesus. The words "Come, and follow me," are exactly the same words used in the call of an apostle (Matthew 9:9); and, as Peter would shortly point out (Mark 10:28), the leaving of all earthly possessions was a requirement the apostles had met.

And come follow me ... Salvation, or eternal life, may be inherited only by those who follow Christ; and, in this requirement of total submission to Jesus' will and of following his commandments, one finds the invariable and universal condition of inheriting eternal life.

Thou shalt have treasure in heaven ... The Lord did not mean that such an act on the part of the young man would in any sense "earn" eternal life. It was not in merely giving away his property that he could have eternal life, for that blessing could come only of following Jesus. If Jesus had permitted this rich young man to become a part of his company of followers without meeting the test which the apostles had all met, it would have had a disastrous consequence in their sacred fellowship; but Jesus was incapable of showing partiality merely because of the wealth of his questioner.