Compare Mark 9:42-50 and Luke 17:1-2. Causing one to sin, or literally to "stumble," meant causing the person to fall from the way of Christ and be damned (as in Jn 6:61; 1 Cor 8:9). Matthew often uses the Greek term in the same manner as here (Mt 17:27; 26:31; compare 5:29-30; 11:6; 13:41; 16:23; Mk 9:42-47).
God Will Avenge the Little Ones, Whom He Favors (18:6-7)
The cruelest legal punishment in Jesus' day was crucifixion, but this image of drowning represents a Roman punishment more horrifying to Jewish hearers than crucifixion and one only rarely tolerated among them (Jeremias 1972:180; for an exception see Jos. Ant. 14.450). When people in a community had much grain to grind, they took it to the community mill, pouring it between an upper and lower millstone. Jesus refers here not to the lighter millstone turned by a woman's hand but to the heavier community kind turned by an ass-heavy enough to take one quickly to the bottom of the sea (Deissmann 1978:81; compare 1 Enoch 48:9). Jesus says this punishment would be an act of mercy compared to what is in store for those who turn little ones from Christ's way-be they arrogant university professors, torturers enforcing Islamic law or gossipers within the church.
Avoiding Hell Is Worth Any Price (18:8-9)
To paraphrase Malcolm X out of context (which is unfortunately how he is usually quoted), we must avoid hell "by any means necessary." Here the image shifts from others as the cause of stumbling to personal responsibility. Because Judaism abhorred self-mutilation (Dalman 1929:227), this is an especially stark image of the cost one must be willing to pay to avoid spiritual death. Enter life was standard shorthand for "enter the life of the coming age" (compare 19:17). The language of losing limbs was reminiscent of the price martyrs paid for their devotion to God (2 Macc 7:11; 4 Macc 10:20). According to a common Jewish belief (as in 2 Baruch 50:2-4; compare 2 Macc 7:11; 14:46), a person with missing members would be resurrected in that form before being restored.