Mat 19:21 If you want to be perfect, go [and] sell what you have, and give to the poor, and you shall have treasure in heaven: and come [and] follow me.

For Christ, perfection (teleios) is one step beyond "righteousness"(dikaiosunê). Righteousness includes concern for worldly things, specifically justice, (which is one of the definitions of the word). In Christ's use of symbols, "righteousness" is a physical state, not an spiritual one. In the Beatitudes, Christ describes the just as people who hunger and thirst for justice and those who are hounded (pursued or persecuted) by others. Perfection is beyond such concerns, beyond the blessings of the Beatitudes.

The message here is not just selling possessions but turning your back on your past accomplishments and your entire past life.

I am always looking for general themes in Matthew's chapters, because every chapter seems to have a central point. The discussion of marriage is a discussion about how difficult marriage can be but that the only alternative is a state of abstenance, which can have a physical cause, or can be chosen for spiritual reasons. Here Christ is offering the same two alternatives. We can live a good life, but if we aspire to something higher, the only higher thing is the spiritual life that leaves physical and emotional concerns behind.

Following Christ here means specifically emulating his life, not just following his teachings.

"Want to be" is from thelo , which means "to be resolved to a purpose" and "to desire."

"Perfect" is from teleios, which means perfect more in the sense of having been perfected, that is, being complete and finished. In animals, it means full-grown. Applied to professionals, it means accomplished or trained. Interestingly, when applied to gods, it means all-powerful, specifically having the ability to fulfull prayers. When applied to prayers, it means being fulfulled or answered. Of numbers and things, it means "finished," "complete." Christ only uses this terms one other time, equating it to the perfection of God in Mat 5:48.

"Sell" is from poleo, which means "to exchange" and "to barter." When this word is applied to people (as it is metaphorically here), it means "to betray" or "to give up."

"What you have" is from huparchonta, which is the past partiple noun form of huparchô, which means "to take the intiative," "to begin." In this form, it means, "that which is in existence," and "the past record."

"Give to the poor" is didomi ptochos, where ptochos means "one who crouches and cringes," and "beggar."

"Treasure" is thêsauros, which means a "storage area" and "treasury." Also meaning a "strong room," "a granary," "a safe," or "a cavern." It is not the valuables themselves, but the place where they are kept, and the basis for our word, "thesaurus."

"Come" is from deuro, which means "hither" and "come on."

"Follow" is from akoloutheô, which means "follow," "go after," "follow a thread of discourse," and "to be consistent with."