One gets the idea from this passage that the work of priests and the ways of the assembly of Israel might be prone to, shall we say, drifting from God’s original intentions (cf. Hebrews 2:1). The subsequent history of Israel is proof positive that drift is the normal tendency of a sinful people; hence the emphasis laid on the priests for guarding the work of the priest and the assembly.

But what were they to guard? Four things.

First, they were to guard the practices and vessels assigned them by the Lord (vv. 21-37). The Lord had given Israel the pattern for where and how they should worship Him. Three times on Mt. Sinai God instructed Moses to make certain that the people followed His exact instructions for building the tabernacle, ordaining the priests, and conducting the business of worship. God knows what He demands of His people, and when He shows them what they must do, He expects them to do it, without compromise or complaint.

So Aaron, the high priest, had a certain work of guarding entrusted to him, and the priests who attended him were also appointed to guard the various vessels and components of the tabernacle as they moved from one place to the next. These items were to be carefully packed, moved, and assembled, just as the practices — sacrifices, offerings, incense, washing, and so forth — that went with the various furnishings of the Tabernacle were also to be scrupulously observed. It fell to the priests of Israel to guard what God had clearly entrusted to them, lest they grow slack in any aspect of their worship and thus show contempt for the holiness of God.

Second, the priests were to guard the worship duties of the assembly — what the text refers to as the assembly’s own “guarding.” The people were instructed as to when they should assemble before the Lord, what their assembling should consist of, and how they should comport themselves (humbly, in fear, and out of love for the Lord). Worship in ancient Israel was hard work, involving precise understandings, careful coordination, diligent attendance, and heartfelt zeal. None of this came naturally; the people had to be instructed, and they had to be guarded to ensure, generation after generation, that their comportment before the Lord, and their practice of worship, which they were to guard, were in accord with His expectations of them. It was the duty of the priests to make sure this was the case.

Third, the priests were to guard one another, that is, to exercise accountability with one another for the proper performance of their assigned duties. Even Aaron had to be “guarded” to make sure he did what was assigned to him. We can imagine as well that the various families of priests—those descended from Gershon, Kohath, and Merari — kept watch over one another, to make sure that each carried out his appointed tasks just so.

Finally, the priests were charged with guarding against the inroads of foreign influence (vv. 10, 38). God strictly forbade any “outsiders” from entering His worship or having anything to do with leading His people in worship. Means were provided, of course, for foreigners and sojourners to participate in the fellowship and covenant of Israel. But “outsiders” were those whose unsanctified, pagan ways were not welcome in the presence of God. It was the duty of priests to guard the assembly and all the protocols of worship against any “outside” infringements.

Guarding in the New Testament
The idea of pastors — and elders — as guardians of the flock of God is clearly present in the New Testament. The word Paul uses in Acts 20:28, which is typically translated “watch over” or “pay careful attention to,” can also just as faithfully be translated as “guard.” Flocks of the Lord’s sheep have been trusted to pastors and elders, and it is their duty to guard them. We hear Paul urging this on Titus in chapter 1 of that epistle; we hear the Lord Jesus defining the work of a shepherd as one who does not flee in the face of danger but stays on to guard the flock (John 10:12-13). Peter’s words to the shepherds of those churches in Asia Minor also carry the implication of guardianship over the flocks of the Lord.