There is no doubt that ἀδόχιμος has its simple force of " reprobate" or "good-for-nothing." I never could find out the difficulty people have found in it. There is no difficulty in being a preacher and oneself rejected, and that is all he says.
I only add that when the wilderness comes in, as he goes on. to introduce it here in Chapter 10, it must be crossed, and the ifs come in. I have not a shadow of doubt that God will keep His own to the end: 1 Cor. 1, John 10, and other passages are far too clear, thank God, to leave a doubt. But we have to be kept, The wilderness forms no part of the counsels of God, but of His ways. No transit is found in Ex. 3;6, or 15. His purposes will be infallibly accomplished; but more-Christ could take the thief to Paradise from off his cross perfected by His one sacrifice, and we can say " Giving thanks to the Father who hath made us meet to be partakers of the inheritance of the saints in light." We are in Christ and Christ in us. There is no " if " there, and no condemnation. But as a general rule God makes us pass through the wilderness. There (Deut. 8) He humbles us and proves us to know what is in our heart, only learning what is in His, that He withdraweth not His eyes from the righteous. He does not suffer our foot to swell, nor our clothes to wax old. What we learn then is not salvation: that is the Passover, the Red Sea, and Jordan. But we learn dependence as our part; we learn to know ourselves and be humble, and we learn the sure watchful faithfulness of God-"kept by the power of God through faith;" but we have to be kept, and, if His, surely will be.
But why kept if we had not need of it? No one can pluck us out of Christ's hand; but why say this if there was not real danger, and keeping of us in it? The wolf catcheth (same word as plucks) the sheep and scattereth them, but cannot catch them out of Christ's hand; and here our responsibility comes in, our dependence on Him, our leaving ourselves to His infallible care; and one is as precious as the other is necessary. Hence, wherever the journey is spoken of, " ifs " are found: when righteousness and our place in Christ, never. In verse 10 he goes on to put this to the Corinthians. Many were delivered out of Egypt and fell in the wilderness; but he does not say many were true believers, and fell in the wilderness. When I can talk of beginning and end, I find if Where one is in Christ, that is not the case; nor if righteousness and justification are spoken of. " By one offering he hath perfected forever." Let me add to this note a remark, wherever falling is spoken of in Hebrews, it is always fatal and hopeless, drawing back to perdition. It is never a fall, but " falling away."
Christ's priesthood in heaven is not for sins; but that we may not sin.