Leviticus 16:2

2.Speak unto Aaron. The sum of the law is, that the priest should not frequently enter the inner sanctuary, but only once a year, i.e., on the feast of the atonement, in the month of September. The cause of this was, lest a more frequent entrance of it should produce indifference; for if he had entered it promiscuously at every sacrifice, no small part of the reverence due to it would have been lost. The ordinary sprinkling of the altar was sufficient to testify the reconciliation; but this annual ceremony more greatly influenced the people’s minds. Again, by this sacrifice, which they saw only once at the end of the year, the one and perpetual sacrifice offered by God’s Son was more clearly represented. Therefore the Apostle elegantly alludes to this ceremony in the Epistle to the Hebrews, where it is said that by the annual entrance of the high priest the Holy Ghost signified,

"that the way into the holiest of all was not yet made manifest, while as the first tabernacle was yet standing,”
(Hebrews 9:8;)

and a little further on he adds, that after Christ the true Priest had come,

"he entered in once into the holy place, having obtained eternal redemption for us.” (Hebrews 9:11.)

Thus the year, in the ancient type, was a symbol of the one offering, so that believers might understand that the sacrifice, whereby God was to be propitiated, was not to be often repeated. That God may inspire greater fear, and preserve the priests from carelessness, He proclaims that His glory should appear in the cloud in that part of the sanctuary where was the mercy seat; for we know that the sign was given from hence to the Israelites, when the camp was to be moved, or when they were to remain stationary. But this testimony of God’s presence should have justly moved the priests to greater care and attention; and hence we may now learn, that the closer God’s majesty manifests itself, the more anxiously should we beware, lest through our thoughtlessness we should give any mark of contempt, but that we should testify our submission with becoming humility and modesty.