This and the preceding verse occur in St. Luke (Luke 17:1, Luke 17:2) in an inverted order. Woe unto the world! The Lord thinks of the deadly evil brought into the world by offences given, such as bad example, unholy lives of Christians, persecutions, scoffs, thoughtlessness—things which lead so many astray.
For it must needs be. While men are what they are, such consequences must be expected. This is not an absolute, but a relative, necessity. Man's heart is evil, his tendencies are evil, temptation is strong.
Satan is active; all these forces combine to bring about a fatal result. Thus St. Paul says (1 Corinthians 11:19), "There must be heresies among you, that they which are approved may be made manifest among you."
So these offences of which Christ speaks are overruled and permitted for wise purposes, that by them the righteous may be proved and purified, and the chaff separated from the wheat. But woe to that man!
Because of this evil principle which is rife in the world, no man is exonerated from the guilt of giving offence. He has free will; he can choose good; he can use the means of grace; he can strengthen his natural weakness, control his perverseness, overcome corruption, by the help of God always ready to be given to them who seek.
The first "woe" is a cry of pity for a world in danger; the second "woe" is a denunciation of the sinner as being responsible for the evil which he introduces. We are all in some sort our brothers' keepers, and are bound to help forward their salvation, and to do nothing which may tend to endanger their souls' health.