In Mark 10:17-31, what did the rich man lack? I tried searching for this on Christianity.SE, but couldn't find anything. Google searches brought up a lot of opinions based on different theologies and opinions. I'm looking for an answer to this out of Scripture. The part I'm talking about is in Mark 10:21:


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


Is the "one thing" simply the "treasure in heaven" Christ is referring to? Or is it something else?

While I'm more looking for a purely Scriptural answer, I'd be interested in getting the Catholic perspective on this.
The one thing he lacked was actually the most important thing - saving faith in God.  That he lacked this is evidenced firstly by his failure to put his trust wholly in Christ to follow Him no matter what, but also by his failure to obey the first (and greatest) commandment evidenced by him prioritising his wealth over the will of God. 


  No one can serve two masters, for either he will hate the one and love the other, or he will be devoted to the one and despise the other. You cannot serve God and money. - Matthew 6:24 ESV