a “How do you think about this? If any man has a hundred sheep, and one of them is gone astray,”
Here in context the hundred sheep represent the new community, His new congregation. They are those who have been gathered out of the lost sheep of the house of Israel (Matthew 9:36; Matthew 10:6; Matthew 10:16; and compare Ezekiel 34:1-16).
The one who goes astray is the one who has been caused to stumble (Matthew 18:6-7), or who has failed to take drastic action over sin (Matthew 18:8-9). The whole picture here is limited to the needs of His own wider group of disciples, His ‘congregation'. A ‘hundred' sheep indicates a complete flock. Up to this point not one was missing.
‘One is gone astray.' Note the emphasis on its oneness. It is out there and alone. It is waiting for someone to come and help it. Shortly, in Matthew 18:15, one will come to help it, and then if necessary two or three. And if that is not enough the whole of the remainder of the congregation (the whole ninety nine). For the whole congregation is a ‘self-help' group with concern for each other, because they love one another, and are aware that they are all forgiven sinners. And they will be acting in the name of the shepherd.