Did the young rich man sin by not following Jesus? We read in the Gospel of Matthew 19:16-30 the story of the rich young man.


  The Rich and the Kingdom of God
  
  Just then a man came up to Jesus and
  asked, “Teacher, what good thing must I do to get eternal life?”
  
  “Why do you ask me about what is good?” Jesus replied. “There is only
  One who is good. If you want to enter life, keep the commandments.”
  
  “Which ones?” he inquired.
  
  Jesus replied, “‘You shall not murder, you shall not commit adultery,
  you shall not steal, you shall not give false testimony, honor your
  father and mother,’ and ‘love your neighbor as yourself.’”
  
  “All these I have kept,” the young man said. “What do I still lack?”
  
  Jesus answered, “If you want to be perfect, go, sell your possessions
  and give to the poor, and you will have treasure in heaven. Then come,
  follow me.”
  
  When the young man heard this, he went away sad, because he had great
  wealth.


Question: Did the young rich man sin by not following Jesus? If he did, was it mortal or venial sin?
The rich young man did not sin by not following Jesus's counsel to a more perfect life, but he did commit the sin of covetousness: "because he was covetous, he went away sad" (Super Evangelium S. Matthæi ch. 19).

Discussing "Whether covetousness is always a mortal sin?" (Summa Theologica II-II q. 118 a. 5 co.), St. Thomas Aquinas says covetousness is a


mortal sin whenthe love of riches becomes so great as to be preferred to charity, in such wise that a man, through love of riches, fear not to act counter to the love of God and his neighbor
venial sin whenthe inordinate nature of his love stops short of this, so that although he love riches too much, yet he does not prefer the love of them to the love of God, and is unwilling for the sake of riches to do anything in opposition to God or his neighbor


If the rich man embraced poverty, this would have helped him increase in charity and perfection.