Charles Spurgeon Commentary Leviticus 16:1-2

"And Jehovah spake unto Moses, after the death of the two sons of Aaron, when they drew near before Jehovah, and died; and Jehovah said unto Moses, Speak unto Aaron thy brother, that he come not at all times into the holy place within the veil, before the mercy-seat which is upon the ark; that he die not: for I will appear in the cloud upon the mercy-seat." — Leviticus 16:1-2 (ASV)

The way into the heavenly places was not yet revealed; the inner shrine, called the holy of holies, was specially guarded from human access. No one could have said in those days, Let us come boldly unto the throne of grace, for only the high priest could approach the mercy seat at all, and he must go within the veil strictly in accordance with the instructions given to Moses by the Lord.

Nadab and Abihu appear to have entered into the presence of God wrongfully. They had probably been drinking, for there was a command afterward given that no priest should drink wine or strong drink when he went into the house of the Lord. God in his righteous anger slew these young men at once, and now, so that no others would intrude into the secret place of communion, a law was given to specify when and how a person might approach God.