Matthew 18:13

“And if so be that he finds it, truly I say to you, he rejoices over it more than over the ninety and nine which have not gone astray.”

The ‘if so be' is a warning against presumption, but Matthew 18:14 cancels out any doubt for the true believer, for the will of the Father cannot be thwarted. A lost sheep may sometimes not be found, but that is never true of one of His Father's little ones. And the point is that when that failing, helpless, foolish sheep is found He rejoice over it (the true disciple who has gone astray and has been found) more than over the ninety nine who did not go astray (the true disciples who remain ever true). This does not mean that He loves the one more than the others. It does not mean that it is more important than the others. It is simply an indication of the compassionate heart of the Shepherd and of how much all His sheep mean to Him (compare here the story of the prodigal son, the father, and the elder brother - Luke 15), and how much they should mean to us. It is in fact what does happen among shepherds in real life. He rejoices over the sheep that is found, and that not one of the sheep so dear to Him has been lost. The rejoicing is because sin and death has been defeated, and a beloved one has been restored. The full number has been made up.