That they should cleanse themselves; partly because the work they were now set upon, though common in its nature, yet was holy in design of it, and had respect unto the sabbath; and partly because the day in which they were to do this was the sabbath day, for the observation whereof they were obliged to prepare and purify themselves. Keep the gates; either,
1. The gates of the temple. But a particular command was superfluous in that case, because it was their constant work and charge to do this, both sabbath days and every day. Or rather,
2. The gates of the city; for of them he spake last, Nehemiah 13:19; and not daring to trust the common porters of these gates, not being able always to employ his men in that work, he committeth the charge of them for the present season, and upon the sabbath days, to the Levites, to whom the care of sanctifying the sabbath did properly belong. According to the greatness of thy mercy; whereby he intimates, that though he mentioned his good works as things wherewith God was well pleased, and which he had promised to reward, yet he neither did nor durst trust to their merit or his own worthiness; but when he had done all, he judged himself an unprofitable servant, and one that needed God's infinite mercy and indulgence to pardon all his sins, and particularly those infirmities and corruptions which adhered to his good deeds.