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In the pastoral setting familiar to his hearers they understand that the ninety nine will stay grouped because they have a lead sheep and generally are all related by birth and tend to hang out together... so the shepherd was able to leave them temporarily to rescue an individual who got into difficulties for whatever reason and became separated... so the shepherd took charge of all the sheep.
Jesus does not use those words in this particular portion of scripture you have quoted from.
Not because of the sheep... but because of "one sinner that repenteth"
It is not stated - it is a generalisation of a familiar pastoral setting familiar to his hearers... which is why Jesus says "What man of you..."
It is only stated that they are sheep... there is no personal pronoun attached to any of the individual sheep.
There is a group identified as - "all the publicans and sinners..."
And another group identified as - "And the Pharisees and scribes..."
And the 100 sheep.
And the shepherd.
And of course Jesus.
.
So, on the one hand, we have this first group, as you mentionned it, who gathers -all the publicans and sinners-. All those were despised, and rejected, even more probably by the second group, that I'll mention afterwards. What makes those people detestable by the Jews and their religious chiefs ? The law. The one who dictate man's life, what he has to do. The one who intercede in the desert, in the time of Moses, the man of faith.
Publicans and sinners are often related in the writings we receive as a unified group. Luke and the others showed, at multiple times, that this was common for them to come to Jesus.
And for this first group, Luke said in his letter that they "drew near unto Jesus" to "hear Him". And even eat with him.
In general, you do not act like that way when you don't agree with someone. On the contrary, it is because those people liked what they already heard of Him, in the first place, that, this time, they drew near unto Him, to hear again from Him.
Because, Him, Jesus, did not condemned them. He received them, and that was propbably "rare" that "those people", you know, the ones we are pointing at sometimes, to be received by someone. But Jesus did, and He talked to them, and ate with them.
And this second group, -the pharisees and scribes- that were specially attached to the law. They have nothing to do with "them", "those dirty sinners and publicans". They were righteous, before themselves at least.
Concerning them, it is not written that they drew near unto Jesus to hear Him, nor eat with him. For them, it is written that they were whispering against Jesus and those people He received. As if they were observing the scene, by far, out of the corner of their eye. And they did not like this "scene". Something was wrong for them. "How can this Jesus receive such people...?" We can assume that they did not receive "those people".
And that is not how God would have act towards them, according to their understanding.
And, as you see it in the text, it is right after those whispers from the pharisees and scribes, that Jesus gives three parables. So here is, in a few lines, the background where the "scene" occured.
And the Pharisees and scribes murmured, saying, This man receiveth sinners, and eateth with them....And he spake this parable unto them, saying,...(Luke 15:2-3)
As in the parable, there is two "groups", and each one represents a "type of sheep". And between those two groups, there is a sheperd, which is none other than Jesus Himself. He is the good sheperd.
I am the good shepherd, and know my sheep, and am known of mine. (John 10:14)
The "lost sheep" represents publicans and sinners. The "ninety and nine" represents the pharisees and scribes".
God is Spirit, and Jesus always talked in a spiritual way, in order to show the Father, who He really is.
Regarding all of this, do you begin to see what the parable is saying...? How can sheeps in the desert not be lost ? Why was there only one of them who was ? Is the desert a "normal" place for a sheep ?
Who came back to the house of the sheperd ?The lost one, or the ninety and nine ? What is
the text saying ?
Who needed repentance, and who did not ? Who did all the way for the lost sheep ?
Did Jesus accused this sheep ? No, He lay it on His shoulders. The sheep did not even had to do the walking. Jesus took her in charge.
But what is said of the others ?
Who receive the care of the sheperd ? The sinner, or the righteous ? The one who put the law in practice, or the one who did not ?
Hugo