Ante-Nicene Fathers, Vol IX:
Origen's Commentary on the Gospel of Matthew.: Chapter XXIII

23.  In What Sense “Necessary.”

Next we must test accurately the meaning of the word “necessity” in the passage, “For there is a necessity that the occasions come,” 5968 and to the like effect in Luke, “It is ‘inadmissible’ but that occasions of stumbling should come,” 5969 instead of “impossible.”  And as it is necessary that that which is mortal should die, and it is impossible but that it should die, and as it must needs be that he who is in the body should be fed, for it is impossible for one who is not fed to live, so it is necessary and impossible but that occasions of stumbling should arise, since there is a necessity also that wickedness should exist before virtue in men, from which wickedness stumbling-blocks arise; for it is impossible that a man should be found altogether sinless, and who, p. 489 without sin, has attained to virtue.  For the wickedness in the evil powers, which is the primal source of the wickedness among men, is altogether eager to work through certain instruments against the men in the world.  And perhaps also the wicked powers are more exasperated when they are cast out by the word of Jesus, and their worship is lessened, their customary sacrifices not being offered unto them; and there is a necessity that these offences come; but there is no necessity that they should come through any particular one; wherefore the “woe” falls on the man through whom the stumbling-block comes, as he has given a place to the wicked power whose purpose it is to create a stumbling-block.  But do not suppose that by nature, and from constitution, there are certain stumbling-blocks which seek out men through whom they come; for as God did not make death, so neither did He create stumbling-blocks; but free-will begot the stumbling-blocks in some who did not wish to endure toils for virtue.

Footnotes

Matt. xviii. 7.

Luke xviii. 1.