John 6:39 and this is the will of him who sent me, that I shall lose none of all that he has given me, but raise them up at the last day.
Yes that is God's will.
These lost sheep are people before the become born again and given the Holy Spirit. After you've been given the Spirit you become..........
Where does it say this teaching only refers to people before they were born again.
4 “What man of you, having a hundred sheep, if he loses one of them, does not leave the ninety-nine in the wilderness, and go after the one which is lost until he finds it? 5 And when he has found it, he lays it on his shoulders, rejoicing. 6 And when he comes home, he calls together his friends and neighbors, saying to them, ‘Rejoice with me, for I have found my sheep which was lost!’ 7 I say to you that likewise there will be more joy in heaven over one sinner who repents than over ninety-nine just persons who need no repentance.
Luke 15:4-7
"My sheep" is a reference to His people that become lost.
JLB