It is impossible - It cannot but happen. Such is the state of things that “it will be.” See these verses explained in the notes at Matthew 18:6-7.
Related Commentaries of Luke 17:1
Luke 17:1-2
1 Then said he unto the disciples,It is impossible but that offences will come: but woe unto him, through whom they come!
2 It were better for him that a millstone were hanged about his neck, and he cast into the sea, than that he should offend one of these little ones.