The Holy Spirit this signifying, that the way into the holiest of all was not yet made manifest, while the first tabernacle was yet standing:
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George Leo Haydock
The Holy Spirit signifying this. Here the apostle begins to tell us in what manner the sanctuary was a figure of things in the new law of Christ. The holy of holies was a figure of heaven, and this prohibition of any one going into it, was to signify that the way to heaven was not to be made manifest, nor to be opened, as long as the former tabernacle and law subsisted; that it was not to be opened till Christ, the high priest of the new testament, first entered, by shedding his blood on the cross, and by his glorious ascension. (Witham)
But when Christ expired, the veil of temple was rent asunder, to show that the way to heaven was now laid open to mankind.
3. Thus far there is no speculation. But from this point he philosophizes and says, Hebrews 9:8 The Holy Ghost this signifying, that the way into the Holiest of all was not yet made manifest, while as the first tabernacle was yet standing. For this cause (he says) have these things been thus ordained, that we might learn that the Holy of Holies, that is, Heaven, is as yet inaccessible. Let us not then think (he says) that because we do not enter them, they have no existence: inasmuch as neither did we enter the Most Holy [place].