The introductory "and" (de in the Greek text, not translated in the NIV) indicates a logical connection with what has preceded. It is inevitable that disciples retard the spiritual progress of others occasionally because none of us is perfect. However that does not excuse personal responsibility when someone causes another to stumble into sin or apostasy (cf. Luke 11:52). It is a very serious offense to hinder the progress of a spiritually immature believer whom Jesus spoke of here as a child (cf. Matthew 18:6). "Woe" recalls Jesus condemnation of the Pharisees in Luke 6:24-26. It indicates the seriousness of this offense. [source][source][source]