Answer: The Epistle is addressed to Hebrews who had professed Christianity. It is an unfolding of the truth of the person and work of Christ the Son of God, in contrast with Judaism. Those that have professed Christianity are looked at as in the wilderness, journeying onward to the rest that remains to the people of God, and their faithfulness to the end is the proof of their reality. All turning away, or falling away, is apostasy: it is giving up the truth that Jesus is the true Savior. The writer puts, himself as one with the rest (See 2:3; 3:6, 14; 4:11).
To sin willfully is to turn away from Christ Jesus; it is to give up Christianity, and such were never saved. The old sacrifices never could put away sins, and there remaineth no more sacrifice for sins, but a certain fearful looking for of judgment and fiery indignation which shall devour the adversaries. They have rejected the only sacrifice that could save them, and God rejects them. Their sin is described in verse 29: they have trodden under foot the Son of God, and counted the blood of the covenant wherewith they were (outwardly) sanctified an unholy thing, and hath done despite unto the Spirit of grace.
This is not a backsliding child of God, but an apostate professor of what he did not possess.
Answer: It means to “set apart.” It is the present activity of the love of Christ to the church, His body, and His bride. “He gave Himself for it,” is the past. “Present it to Himself,” is the future. But now He is taking us out from the world, and setting us apart for Himself, and cleansing us by the washing of water by the Word. (Compare Psa. 119:99BETH. Wherewithal shall a young man cleanse his way? by taking heed thereto according to thy word. (Psalm 119:9)). The way He does it is by exercising our souls in the truth that we now belong to Him who is rejected down here, but glorified up there. This causes us to walk as heavenly citizens. As we take in the truth that we are members of His body, part of His bride, and it gets its right place in our souls, it separates our affections from the world, and places them on Him.
Thus in verse 25, we see what He did; in 26, what He is doing; and in 27, what He is going to do for us,