First of all, I’d like to mention some interesting quotations from commentators, that help us to paint a picture of the entrance of the high priest in the most holy place of the Tabernacle.
NET Bible notes (on Lev 16:2; bold is mine): “tn Heb ‘into the holy place from house to the veil-canopy’. In this instance, the Hebrew term ‘the holy place’ refers to ‘the most holy place’ (lit. “holy of holies”), since it is the area ‘inside the special curtain’ (cf. Exod 26:33-34). The Hebrew term פָּרֹכֶת (parokhet) is usually translated ‘veil’ or ‘curtain’.” […] “tn Heb ‘to the faces of the atonement lid’. The exact meaning of the Hebrew term כַּפֹּרֶת (kapporet) here rendered ‘atonement lid’ is much debated. The traditional ‘mercy seat’ (KJV, ASV, NASB, NRSV) does not suit the cognate relationship between this term and the Piel verb כִּפֶּר (kipper, ‘to make atonement, to make expiation’). The translation of the word should also reflect the fact that the most important atonement procedures on the Day of Atonement were performed in relation to it. Since the LORD would ‘appear in the cloud over the atonement plate’, and since it was so closely associated with the ark of the covenant (the ark being his ‘footstool’; cf. 1 Chr 28:2 and Ps 132:7-8), one could take it to be the place of his throne at which he accepts atonement. See J. Milgrom, Leviticus (AB), 1:1014; J. E. Hartley, Leviticus (WBC), 234-35; and R. E. Averbeck, NIDOTTE 2:691, 699. Cf. NIV ‘the atonement cover’; NCV ‘the lid on the Ark’; NLT ‘the Ark’s cover – the place of atonement’.
Keil & Delitzsch (bold is mine): “The death of Aaron’s sons, as a punishment for wilfully ‘drawing near before Jehovah’, was to be a solemn warning to Aaron himself, ‘not to come at all times into the holy place within the v[e]il, before the mercy-seat upon the ark’, i.e., into the most holy place (see Exo 25:10.), but only at the time to be appointed by Jehovah, and for the purposes instituted by Him, i.e., according to Lev 16:29., only once a year, on the day of atonement, and only in the manner prescribed in Lev 16:3., that he might not die. ‘For I will appear in the cloud above the capporeth.’ The cloud in which Jehovah appeared above the capporeth, between the cherubim (Exo 25:22), was not the cloud of the incense, with which Aaron was to cover the capporeth on entering (Lev 16:13), as Vitringa, Bähr, and others follow the Sadducees in supposing, but the cloud of the divine glory, in which Jehovah manifested His essential presence in the most holy place above the ark of the covenant.
Because Jehovah appeared in this cloud, not only could no unclean and sinful man go before the capporeth, i.e., approach the holiness of the all-holy God; but even the anointed and sanctified high priest, if he went before it at his own pleasure, or without the expiatory blood of sacrifice, would expose himself to certain death. The reason for this prohibition is to be found in the fact, that the holiness communicated to the priest did not cancel the sin of his nature, but only covered it over for the performance of his official duties, and so long as the law, which produced only the knowledge of sin and not its forgiveness and removal, was not abolished by the complete atonement, the holy God was and remained to mortal and sinful man a consuming fire, before which no one could stand.”
Alter’s Bible note on Lev 16:2 (bold is mine): “Only on this most sacred day [of Atonement] […] is the high priest granted access to this dangerous space, and only when he follows a strict regimen of dress and action.”
The Cambridge Bible (bold is mine): “the mercy-seat] Heb. kappôreth, here and in Lev 16:13-15, a solid gold plate of the same size as the top of the ark (2½ by 1½ cubits), to which the two cherubim were fixed, as described in Exo 25:17-21. It was the place where the Lord appeared (Lev 16:2); from which He gave His commands (see note on Lev 1:1); the most holy spot in the most holy place, the ‘footstool’ of the Lord who sitteth enthroned upon the cherubim (1Ch 28:2; Psa 99:5, cp. Lev 16:1; Psa 132:7, cp. Lev 16:5). The Heb. word is formed from kipper, to make propitiation, and means that which propitiates. The Gk. ἱλαστήριον exactly corresponds, and from the Vulg. propitiatorium the word ‘propitiatorie’ was used in Wiclif’s translation. This word is the best English equivalent for the Heb., and indicates the nature of the solemn rite performed within the veil on the Day of Atonement. As ‘oratory’ is the place of ‘oration’ or prayer, so ‘propitiatory’ in the sense of ‘place of propitiation’ would fitly express the Heb. word which is rendered in EVV by ‘mercy-seat.’ See note on Exo 25:17 (C.B.); Art. Mercy-seat in Enc. Bib.; and Art. Tabernacle in HDB. iv. 665 a. The mercy-seat is described as ‘upon the testimony’ in Lev 16:13. ‘The testimony’ is the name given to the two tables on which the Ten Words were written, so called because they contain the ‘testimony’ or witness of God’s will for man. See note on Exo 25:16, and Intr. to Pent. App. II. pp. 221 f.”
After this roundup of scholars clarifications, we must return to the Bach-asked direct questions.
As in other instances, this questions can be solved through a correct understanding of Hebrew particles. In this case, כי (KI), according a NET Bible’ note (on Gen 5:24) denotes “a primitive particle (the full form of the prepositional prefix) indicating causal relations of all kinds, antecedent or consequent”.
The manner to translate כי (KI) depends on the (omnipresent) context, as well as from the logical wording flow.
Anyway, this particle can be rightly translated as ‘because, forasmuch as, inasmuch, since as’, and alike.
Gen 5:24 (where כי is translated as ‘because’: DRB, Good News Bible, ISV, Louis Segond [French transl.], Reina-Valera [Spanish transl.].
Gen 32:12 (כי sense: ‘due to the fact’, ‘because’), ‘for’, as in a lot of translations.
2 Sam 19:8 (כי sense: ‘since’), ‘for’, as in a lot of translations.
So, the final sense is:
Since [כי (KI)] the presence of God was over the Ark (placed inside the Holy of Holies), was not allowed to the high priest to enter there as he prefer - under death penalty - but only on a time and by a manner fixed by God.
John Gill aptly wrote (bold is mine):
“[…] this one would think should be a reason why he should not die, when he came into the most holy place, because there was the mercy seat, and Jehovah on it: and besides the cloud of incense on it, he went in with, for so many understand by the cloud, the cloud of incense: thus Aben Ezra says, the sense is, that he should not enter but with incense, which would make a cloud, and so the glory not be seen, lest he should die: and Jarchi observes, that the Midrash, or the more mystical and subtle sense is, he shall not go in but with the cloud of incense on the day of atonement; but the more simple meaning, or plain sense of the words is, as the same writer notes, that whereas he did continually appear there in the pillar of cloud; and because his Shechinah or glorious Majesty is revealed there, he is cautioned not to use himself to go in, i.e. at any time; with which agrees the Targum of Jonathan,’for in my cloud the glory of my Shechinah, or divine Majesty, shall be revealed upon the mercy seat’. And this being the case, such a glory being there, though wrapped up in a cloud and thick darkness, it was dangerous to enter but by divine order.”
Addenda for the Christians readers:
It seems to me that John of Zebedee – in the Revelation – linked this miraculous phenomenon causing a supernatural light (the Shekinah, named so by non-biblical writers) inside the Holy of the Holies with the vision he had about a town ‘in which the night does not exist’. As the Holy of the Holies was illuminated entirely by God, so the lighting of ‘New Jerusalem’ depends on God (Rev 21:22-25).
A little (final) gem: it seems that the text did intend a pun between the two pivotal terms of this Leviticus’ passage. In fact, we find here פרכת (PRKT, the ‘veil’, or ‘curtain’), as well as כפרת (KPRT, the ‘lid of propitiation’), where the only difference (devoid the Masoretes’ diacritical system) is a metathesis, that is a transposition of the consonant kaf from the third position (inside the term) to the first position.
I hope this answer to your dilemma.