“Who devour widows' houses, and for a pretence make long prayers.”
We do not know quite how they devoured widow's houses. They were not supposed to receive payment for teaching. But they could soon find themselves idolised, and the worst would then no doubt be open to receiving munificent gifts on some pretext, and to the misusing of hospitality offered to them, along with their friends, by fairly wealthy widows, or even less wealthy ones, until the wealth ran out. Or it may be that they wangled their way into acting in trust for widows on behalf of their deceased husbands, in the process wasting their goods like the unrighteous steward. Jesus clearly knew of a number of widows who had been impoverished by the greed of the Scribes.
Furthermore they added to their sins by ostentatious praying. While behaving in such ways they would spend much time in prayer in order to impress people with their sanctity, and pray openly in public, both in the synagogues and in the marketplaces (Matthew 6:5) and in the Temple (Luke 18:10), doing it openly and for longer periods than normal with a view to gaining people's trust. The thought among the less knowing would be that anyone who prayed like that must be trustworthy. It was only Jesus Who knew men's hearts fully for what they were.
“These will receive greater condemnation.”
And these will receive greater condemnation because they have abused the trust given to them, and the trust that others have in them (compare Luke 17:1-2). In what way would it be greater?
1). It will be greater than the condemnation of Chorazin and Bethsaida, greater than that of Capernaum (Luke 10:13-15), because they had received greater privileges and had failed to take advantage of them in order to become truly spiritual (compare Luke 12:47).
2). It will also be greater than the high estimation that they have of themselves.
3). It will be greater even than their hypocrisy.