Who are they?
Who are those who are well? Literally they are those who are healthy, those who do not need a physician. In Greek they are defined as ischyontes, "those who have strength, force, power." Ischyontes is the present participle of the verb ischyo, "I am strong, I am healthy, I can, I manage". Ischys is "force", a great physical strength, the overall power of a human being.
Who are the sick? They are those who "hold onto badly". In the Greek version of the Gospel the sick are defined as echòntes kakòs. Echòntes is the present participle of the verb ècho and refers to possession, to keep and hold tight in one's hand, to hold steady, thus also to hold on through thoughts, to hold within, to hold in consideration or regard; kakòs is an adverb that means "badly", "with malice", "in a harmful way", "evil". Therefore echòntes kakòs, the sick, can also be translated as "those who hold in their thoughts badly", "those who hold onto malice".
Who are the righteous? The righteous person respects what is required by law, by rule. In Greek dìkaios is a fair person, who shares and pays rightly, he/she is the justified, the one who cancels and, in turn, whose debt is canceled; he is a free man, a man who has been granted an amnesty.
Who are the sinners? The sinner, in the precise meaning of the Greek term amartolòs, is one who misses the target. He/she is one who is lost, stray, who wanders off the path. The sinner is in error with respect to the fundamental decisions about his/her aim in life; he/she is one who misses the vital target of his/her existence, focusing on no vital targets, thus missing the primary one. Therefore he/she is off course, he/she is lost and is losing continuosly. For Jesus, the sinner is one who is in debt in terms of love and justice because of an error, a profound spiritual error: he/she has made a bad decision with regard to the object of his/her aims and of his/her total dedication. The sinner chooses the wrong spiritual, intellectual and emotional target. The sinner is one who makes a mistake in choosing the main objective of life itself.
Jesus came to call the sinners and not the righteous. The verb "to call" here has a surprisingly uncommon meaning. Indeed, the Greek kalèo means to summon, to invite; it is the verb used to invite to a wedding, to summon to very important events, it is also the verb used to impose a name. Its etymology goes back to the concept of the messenger, indeed the Akkadian term kallu means "official messenger responsible and bound to call people to their task-job-office". Jesus came to call and impose a new name, to offer a new task, to summon to new life those who recognize that they are sinners and debtors. Jesus' proposal is perfectly useless, mute and meaningless to those who are full of righteousness.