If Exodus 25-27 is a literal, eyewitness report of what Moses was told by God to do, then this must remain a mystery. However, the Documentary Hypothesis holds that the Book of Exodus was written over a period of centuries during the early to middle parts of the first millennium BCE. While the old Documentary Hypothesis is no longer seen as immutable, this table shows a simplified summary of Priestly Source ('P') contributions to the Book of Exodus (and the Pentateuch as a whole).

The Priestly Source, a group credited with authorship of Exodus chapters 25-31 and 35-40, was very concerned with minutiae - numbers, lifespans, detailed genealogies and such exquisite details as the building of the Tabernacle. P's goal was largely to provide a basis for its laws and priestly conventions especially in the post-Exilic period when, with no king to share power, the Jews were coming under theocratic rule. The priestly class wanted proper respect for themselves and demanded that the people of Judah place a high priority on Temple reconstruction, with the most lavish sacred items and furnishings possible. The description of Moses' Tabernacle would set the standard the priests expected.

We can also look at the broader context of the Pentateuch as a whole, to better understand the Priestly agenda. Barat Ellman (Memory and Covenant, pages 124-125) says of the Book of Numbers, the first ten chapters outline the organisation of the community as envisaged by the priesthood, then the middle section offers a "collective memory" of disastrous events in the wilderness to justify priestly authority. On page 126 we again see that the sight of the sacrificial altar in the Tabernacle was to remind Levites and Isrraelites alike of priestly authority. Moshe Weinfeld (Deuteronomy and the Deuteronomic School, page 185) says the bulk of the laws in the Priestly source depend on the divine tabernacle and all that relates to its construction and the ministrations performed in it.

This answer will draw on two published Christian commentaries on the construction of the Tabernacle:

1. The anonymous book of Hebrews (traditionally attributed to Paul) in the New Testament, a first century Christian work, which provides a broad-strokes exposition of the Tabernacle as referring to heaven and Christ's work there.
2. Arcana Coelestia ("Secrets of Heaven") by Emanuel Swedenborg (1688-1772), published during the Age of Enlightenment (London: 8 Latin volumes, 1749-1758), which provides a detailed verse-by-verse commentary on all of Genesis and Exodus, including the later chapters of Exodus outlining the construction and use of the Tabernacle.

Though Swedenborg does not explicitly refer to Hebrews in his commentary on the Tabernacle, references to Hebrews elsewhere in his published and unpublished works make it clear that he looked to Hebrews as a source and confirmation for his spiritual understanding of the Tabernacle and its furnishings and rituals.

For contemporary Christians, then, providing the foundation in the book of Hebrews will offer a greater understanding of the detailed superstructure Swedenborg built on that foundation many centuries later.

The Foundation: The Book of Hebrews

The anonymously written Epistle to the Hebrews is a literary and theological tour de force, offering a grand overview of the vast sweep of Hebrew scriptures as foreshadowing and symbolizing the atoning and saving work of Christ. In this book, Jesus Christ is presented as the great High Priest, whose sacrifice once and for all accomplishes eternally what the earthly sacrifices in the Tabernacle could never achieve.

Hebrews chapters 8 and 9 focus especially on the Tabernacle and the service of the priests there as an earthly symbol of Christ as the High Priest in heaven.

Chapter 8 builds on the theme developed in Chapter 7: that of Christ as the great High Priest in the order of Melchizedek. This is the context in which the earthly and heavenly Tabernacles are woven into the commentary:

1 Now the main point in what we are saying is this: we have such a high priest, one who is seated at the right hand of the throne of the Majesty in the heavens, 2 a minister in the sanctuary and the true tent that the Lord, and not any mortal, has set up. 3 For every high priest is appointed to offer gifts and sacrifices; hence it is necessary for this priest also to have something to offer. 4 Now if he were on earth, he would not be a priest at all, since there are priests who offer gifts according to the law. 5 They offer worship in a sanctuary that is a sketch and shadow of the heavenly one; for Moses, when he was about to erect the tent, was warned, "See that you make everything according to the pattern that was shown you on the mountain." (Hebrews 8:1-5, italics added)

Here the author introduces the concept of the earthly Tabernacle as "a sketch and a shadow" of the heavenly Tabernacle, which is "the true tent that the Lord, and not any mortal, has set up."

And the instruction to Moses to follow precisely the pattern of the Tabernacle shown him on the mountain, quoted from Exodus 25:40, is referenced as confirmation that the earthly Tabernacle was built according to a divine pattern, such that its very structure was a shadow and model of heavenly things.

This theme is taken up more specifically in Chapter 9, which expands on the sacrificial worship performed in the earthly Tabernacle, and its relationship to the true, divine sacrifice of Jesus Christ—which is the reality behind the earthly shadows. Jesus Christ is himself the both the High Priest and the sacrifice, and the Tabernacle is the heavenly "structure" in which the sacrifice is made.

Verses 1-10 provide a thumbnail sketch of the various parts and functions of the "earthly sanctuary" (the Tabernacle of Moses) described in Exodus 25-30.

Verses 11-28 then apply these details of the earthly sanctuary to the heavenly sanctuary: the spiritual realities of which they are only a sketch and a shadow.

11 But when Christ came as a high priest of the good things that have come, then through the greater and perfect tent (not made with hands, that is, not of this creation), 12 he entered once for all into the Holy Place, not with the blood of goats and calves, but with his own blood, thus obtaining eternal redemption. (Hebrews 9:11-12, italics added)

The Tabernacle that Christ enters, then, is not the earthly one in which the Levitical priesthood offered sacrifices of bulls and goats, but rather a greater, heavenly Tabernacle not made with (human) hands:

23 Thus it was necessary for the sketches of the heavenly things to be purified with these rites, but the heavenly things themselves need better sacrifices than these. 24 For Christ did not enter a sanctuary made by human hands, a mere copy of the true one, but he entered into heaven itself, now to appear in the presence of God on our behalf. (Hebrews 9:23-24, italics added)

Here the author of the Hebrews makes his clearest and most definitive statement that the physical Tabernacle whose pattern was given to Moses on Mt. Sinai was an earthly representation of a greater spiritual tabernacle, which is "heaven itself," into which Christ entered in making his sacrifice on our behalf.

To sum up, in the Epistle to the Hebrews, the Tabernacle of Moses is presented as a representation in earthly form of greater spiritual and heavenly realities. And though its author is not at leisure to go into great detail (see Hebrews 9:5b), based on the overview he provides of the heavenly meanings of some of the major parts and furnishings of the Tabernacle, the clear implication is that every detail of the earthly Tabernacle is a sketch and a shadow of a greater heavenly reality.

The answer provided to this question in the Book of Hebrews, then, is this:

God gave such a specific and detailed plan for the earthly Tabernacle because every detail sketches and shadows—or in somewhat more modern terminology, symbolizes and signifies—greater realities of heaven and of Christ's atoning work in heaven as the great High Priest and the eternal divine sacrifice on our behalf.

A Superstructure: Swedenborg's Commentary on the Tabernacle in Arcana Coelestia

This reading of the Epistle to the Hebrews is the foundation upon which Emanuel Swedenborg builds his extensive and detailed superstructure of commentary on the structure and function of the Tabernacle of Moses. That commentary occupies almost the entire final Latin volume of the eight volumes of his magnum opus, Arcana Coelestia ("Secrets of Heaven")

In introducing his commentary on Exodus 25, where the instructions for the building and use of the Tabernacle begin, Swedenborg writes:

This chapter describes the offerings for the tent, for the tables there, and also for Aaron's garments. It then goes on to give the directions for making the ark, the table for the loaves, and the lampstand, which were to represent the heavens, where the Lord is, and all the heavenly and spiritual realities which are from the Lord there. The dwelling-place itself represented heaven, the ark there represented the inmost heaven, and the Testimony or the law within the ark represented the Lord; the loaves of the Presence on the table together with the lampstand represented the heavenly realities from the Lord, and Aaron's garments represented the spiritual realities from him, in the heavens. (Arcana Coelestia #9455, italics added)

Here Swedenborg indicates, in compact form, that his commentary will be based on the same premise presented over a millenium and a half earlier in the Book of Hebrews: that the Tabernacle of Moses was an earthly representation of heaven itself, where the Lord (Christ) is; and that each of its details represents some specific detail about heaven—all of them flowing from the presence of the Lord there.

Building on the foundation provided by Hebrews, Swedenborg proceeds to build an extensive spiritual superstructure, expanding on the heavenly meaning of every single detail of the physical structure described in the later chapters of Exodus.

Obviously we cannot provide all of that detail here. Swedenborg's commentary on the Tabernacle occupies the bulk of two volumes in most English translations. However, here is a sample in which he provide a summary of the meanings involved in the construction of the Ark of the Covenant:

"And they are to make an ark" means the inmost heaven. "Of acacia wood" means righteousness. "Two cubits and a half shall be its length" means all so far as good is concerned. "And a cubit and a half its breadth" means what is complete so far as truth is concerned. "And a cubit and a half its height" means what is complete so far as degrees are concerned. "And you shall overlay it with pure gold" means that all those aspects must be founded on good. "On the inside and on the outside you shall overlay it" means everywhere. "And you shall make on it a rim of gold round about" means a border of good, serving to defend them from the approach of evils and the harm these can do. "And you shall cast four rings of gold for it" means Divine Truth joined to Divine Good, which lie in every direction round about. "And put them on its four corners" means firmness. "And two rings shall be on one side of it and two rings on the other side of it" means the marriage of truth to good and of good to truth. "And you shall make poles of acacia wood" means power derived from this. "And overlay them with gold" means good in all directions. "And you shall put the poles into the rings" means the power of the Divine sphere. "On the sides of the ark" means in the outermost parts. "To carry the ark on them" means thereby bringing heaven into being and keeping it in being. "The poles must be in the rings of the ark" means that the power must be established by the Divine sphere of goodness and truth. "They must not be removed from it" means everlastingly and unalterably so. "And you shall put into the ark the Testimony" means Divine Truth, which is the Lord in heaven. "Which I shall give you" means its representative. (Arcana Coelestia #9484)

If all of this seems rather cryptic, that's because this is only his summary of the spiritual meanings in the details of the construction of the ark provided in Exodus 25:10-16. He then goes on to provide a full explanation of each detail of the construction of the ark summarized above. This occupies the next twelve or thirteen pages in the (somewhat archaic) English translation quoted above.

In Arcana Coelestia Swedenborg provides a similarly detailed commentary on every part and detail of the construction of the Tabernacle, its furnishings and implements, the priestly garments, the incense, the bread of Presence, and all of the other instructions and details related to the Tabernacle and its service that are given in the remaining chapters of Exodus.

Though it would be possible, and even enjoyable (for me, anyway) to go on at length providing further tie-ins of Swedenborg's superstructure to the foundation laid in the Book of Hebrews, that is unnecessary for the purposes of answering this question.

The answer offered in Swedenborg's extensive commentary on the details of the Tabernacle is this:

In the narrative of Exodus, God was so specific and detailed in the instructions to Moses about how to build the Tabernacle because every detail of the construction and proper use of the Tabernacle represents some specific detail of the greater Tabernacle of heaven, in which Christ as the great High Priest achieves atonement and salvation for us once and for all by his spiritual and divine sacrifice in that heavenly Tabernacle.

A number of different answers to this question have been given by Christians through history. Broadly speaking, the reasons for the extensive detail given by Moses might be categorized as follows: (1) to foreshadow future events and entities, (2) to demonstrate the importance of religious worship, and (3) to display God's glory.

Foreshadowing

Christians widely recognize, based on passages Hebrews 9, that some aspects of the tabernacle are types of New Testament realities. However, from the days of the church fathers, many have attempted to seek out a spiritual interpretation or foreshadowing for every detail of the Tabernacle. Gregory the Great, for example, sees the Ark of the Covenant's features as significant:

What but the holy Church is figured by the ark? To which four rings of gold in the four corners are ordered to be adjoined, because, in that it is thus extended towards the four quarters of the globe, it is declared undoubtedly to be equipped for journeying with the four books of the holy Gospel.¹

Similarly, the two cherubim, Gregory says, signify "the two testaments," since the word means "fullness of knowledge."²

Examples of this sort of interpretation apparently abounded in the early church, since Origen, no stranger to allegorical exegesis, warned about some who go too far:

When the (description of the) fitting out of the tabernacle is read, believing that what is written is a type, they seek to adapt what they can to each particular related about the tabernacle,— not being wrong so far as regards their belief that the tabernacle is a type of something, but erring sometimes in adapting the description of that of which the tabernacle is a type, to some special thing in a manner worthy of Scripture.³

Importance of worship

Some argue that the level of detail here is an indication of the importance of worship to God. Matthew Henry contrasts the amount of detail mentioned in the Creation account:

He that gave us no account of the lines and circles of the globe, the diameter of the earth, or the height and magnitude of the stars, has told us particularly the measure of every board and curtain of the tabernacle; for God’s church and instituted religion are more precious to him and more considerable than all the rest of the world.⁴

Matthew Poole reasons that the additional detail was necessary here because man on his own could not deduce what was necessary to properly worship God:

the light of reason implanted in all men, gives him greater help in the discovery of moral and judicial things than in ceremonial matters, or in the external way and manner of God’s worship; which is a thing depending wholly upon God’s institution, and not left to man’s invention, which is a very incompetent judge of those things, as appears from hence, because the wittiest men, destitute of God’s revelation, have been guilty of most foolery in their devices of God’s worship.⁵

God's glory

A third explanation is that the detailed instructions ensured that the tabernacle best displayed God's glory. Coffman writes:

Despite the fact of the tabernacle, where God would dwell (in a figure) with his people, being in essence a portable tent with an enclosure about it, its holiness was emphasized in the fact that only the choicest materials were to be used in its construction.⁶

Keil and Delitzsch argue that the specifics had to come from God, not man:

If the sanctuary was to answer its purpose, the erection of it could not be left to the inventive faculty of any man whatever, but must proceed from Him, who was there to manifest Himself to the nation, as the Holy One, in righteousness and grace.⁷

Young likewise sees the completed tabernacle as something that would cause the world to praise God:

These Israelites, when all was finished according to the pattern in the mount, had then something to show which would make an impression on men of the right sort in the outside world. Here was an answer to the question, "Where is now your God?" Visible he himself is not; but here is a dwelling-place not in anything constructed after art and man's device, but entirely of Divine direction. All our institutions are nothing unless we can trace them to the inspiration and control of God.⁸

Summary

Different Christians will tend to emphasize one or another of these views, at the expense of the others, but it seems likely that there is some truth to each of them. Christians can usually agree that the tabernacle points to Christ, and that it indicates the importance of the worship and glory of God.

References

1. Pastoral Care 2.11
2. Homily 25
3. First Principles, Book 4
4. Complete Commentary
5. Annotations
6. Commentaries
7. Commentary
8. Pulpit Commentaries