Now we arrive at once at a question enormously emphasized by the Holy Ghost: against whom went forth the oath of exclusion?"For who, when they heard the actual voice of God did provoke?" not Egyptians... not the Seven Tribes of Canaan,... not Moab nor Amalek, none of them were ever shut up to Jehova, severed from all the world in a desert as the sole people of God:  "nay, did not all they that came out of Egypt" - Israel, under passover blood and through Red Sea baptism.  "And with whom was He displeased forty years? was it not with them that sinned"- as only believers can sin; that is, against privilege and light- "whose carcases fell in the wilderness?"  The carcasses were proof of the oath: they so pampered the body, that mere bodies they became, repaing corruption. 
"And to whom swear He that they should not enter into His rest" - against whom went forth God's oath of exclusion - "but to them that were disobedient?"  a justified but an unsanctified people. 

But what exactly was the sin which provoked the oath? "We see that they were not able to enter in because of unbelief" (Heb. 3:19 "So we see that they could not enter in because of unbelief.").  But unbelief in what?   Israel's whole wilderness standing was on faith.   "BY FAITH" [Moses] kept the passover, and the sprinkling of the blood;"BY FAITH they passed through the Red Sea as by dry land" (Heb. 11:28).  The unbelief was not in fundamentals: we are never told that Israel doubted their salvation from Egypt, and their ransom by blood:  on the contrary, the exact moment and cause are revealed when God's oath left His lips!  The rejected report of the godly spies completed Israel's unsanctification: "because all these men have tempted me THESE TEN TIMES, they shall not see the land" (Num. 14:22).   "they dispised the pleasant land, therefore He lifted up His hand [to swear]".  (Psalms 106:24).  Israel strongly revolted against God's picture of the future, and the corresponding demands made upon them: it was the partial unbelief of the regenerate (those who had been regenerated) "IN THIS thing ye DID NOT BELIEVE the Lord your God" (Deut. 1:32).  So Paul says:-- "The word of the hearing" (which was the report drawn up by Caleb and Joshua concerning the Kingdom beyond Jordan) "did not profit them, because it" --(not the good news that pointed back to the blood, but the good news that pointed forward to the crown; not the gospel of Grace, but the gospel of the Kingdom) "was not mixed with faith in them that heard" (namely, the people of God.  God gives us not only facts backward to believe, but facts forward: never to believe the facts backward is to be lost; not to believe the facts forward is for a child  of God to drift at once into sin, and to incur the peril of the oath of exclusion). 

So the Apostle closes on the clearest warning to the Church of God. "We which have believed do enter" (are entering; all believers are runners with the goal ahead, without having yet breasted the ribbon) "let us therefore" (Paul even includes himself here) "fear"
(for great and frequent have been the falls of eminent saints) "lest haply a promise" (but conditional because the oath of inclusion (Ex. 13:5) on a failure in sanctity, was met by the counter-oath of exclusion) "being left of entering into His rest, any one OF YOU" (three times the Holy Spirit empties this torpedo into the breast of the Church, the holy brethren who are partakers of the heavenly calling (Heb. 3:1), "should seem to have come short of it".  The mere suspicion of failure, even though it may not be fully justified, for man's judgment is necessarily fallible, is a thing to be earnestly dreaded.  "Let us therefore give diligence" (earnestly strive) "to enter into the rest, that no man [none of you] fall after the same example of DISOBEDIENCE" -- an unholy walk springing from a secret unbelief.
The wilderness is a corridor to the palace, but we may so slip in the corridor as to miss the palace.

One New Testament passage decisively proves (and a multitude confirm) the possibility of exclusion: let us examine it. 

As, in the regenerate, the current of being sets towards good, and the evil is a backwater; so, in the unregenerate, the current of being sets towards evil, and the effort after good is a backwater: and this is always the criterion of regeneration.  "He that doeth righteousness is righteous: he that doeth sin is of the devil" (1 John 3:7,8).  "Faith alone saves; but faith which is alone is not faith" (Luther).  Yet it is also certain that the regenerate can sin deeply, and die in such sin.  For as an example, three facts decisively establish the regenerate nature of the incestuous brother whom the Holy Ghost has made a perpetual and conclusive proof. 
(1) Excommunication was to deliver his flesh, but not his spirit, to Satan: Satan might touch his body, like Job's, but not his soul: "that the spirit may be SAVED in the day of teh Lord Jesus" (1 Cor. 5:5).  Now the destruction, like Ananias's, might be immediate and yet his salvation was assured: therefore he was regenerate before excommunication. "When we are judged" (even unto death (1 Cor. 11:30) "we are chastened of the Lord, that we may not be condemned with the world" (1 Cor. 11:32). 
(2) Paul sharply limits the jurisdiction of the Church to believers! "do not ye judge them that are within, whereas  them that are without God judgeth?  Put away the wicked man" (pass sentance on him) "from among yourselves."  The right to judge unbelievers, Paul says belongs solely to God: therefore the incestuous brother, judged by brother, if excommunicated at all, was promptly restored: because in his 2nd Epistle Paul says "forgive him and comfort him; confirm your love toward him" (2 Cor. 2:7).  This is absolutely decisive.  The sharp discipline has severed him from his sin: acting under an inspired command the Church restored him to full fellowship, as a living member of Christ.   Therefore a believer can so sin, and has: and since there may be destruction of the flesh, can also die in it.  Just as no natural deaths (not even Moses and Aaron) are recorded in the Wilderness, and all who were slain for fornication, etc., were already thereby excluded from Canaan, so it is with the excomunicate committed to Satan for the destruction of the body. 
But a fact of overwhelming decisiveness still remains.  Paul states that the identical sin might permeate the whole assembly.  "Know ye not that a little leaven leaveneth the WHOLE LUMP?"   Was the "whole lump" all good dough, or half bad?  Was the assembly regenerate throughout or not?  "Purge out the old leaven, that ye may be a new lump" (fresh pure dough throughout) "even as ye ARE unleavened" : now keep so, Paul says, and if any leaven returns, purge it out, to keep the lump new.  Because fornication, and also the other immoralities named might spread through the entire Church: "know ye not that a little leaven leaveneth the whole lump?"  So far from Paul regarding the incestuous brother as no believer, because of his fornication, he asserts exactly the reverse-- that, unless drastic measures purge the Body, immoralities may contaminate the whole.   No disciple is immune from peril;  and Paul therefore devotes the rest of the chapter to proving how great a sin fornication is in one indwelt by the Holy Ghost.  "Shall I then," (because the sin is possible even to an apostle) "take away THE MEMBERS OF CHRIST and make them members of a harlot?" (1 Cor. 6:15).  If Paul has unbelievers in mind, then he warns them of a sin which they cannot commit because to take the members of Christ, and make them members of a harlot, is not possible to those not in the body of Christ. It can only be possible to one IN Christ.  So Paul speaks throughout the passage, solely of the members of the Body of Christ. 
Therefore, it is certain that believers can commit such sins.  It is also certain that some in Corinth did.  It is also certain that all such are to be excommunicated:

Paul now unfolds the tremendous revelation that disciples so unclean as to be shout out of the Church, must also be shut out of the Kingdom;...
 
 

For what is the catelogue of excommunication? Fornicators, idolaters, covetous, drunkards, revilers, extorsioners (1 Cor. 6:9).  And what is the catelogue of exclusion?
"Ye yourselves do wrong"  (at what peril?) "Know ye not that wrong-doers shall not inherit the kingdom of God?  Be not deceived neither fornicators, nor idolaters, [4 new sins are now added, 3 an expansion of fornication, and 1 an expansion of covetousness: exclusion from the kingdom is a wider thing than excommunication]nor covetous, nor drunkards, nor revilers, nor extortioners [notice that each excommunicating sin is also an excluding sin] shall inherit the kingdom of God".  It is the same list! The justly excommunicated will be the infallibly escluded.  For "whose soever sin ye forgive [the incestuous brother's] they are forgiven unto them; whose soever sins ye retain [excommunication is what God has commanded] they are retained" (John 20:23)
(See also Matthew 18:18... "what things so ever ye shall bind on earth shall be bound in heaven". )

"Such were some of you; but ye were washed" [through blood and water] "but ye were sanctified" [set apart for God as hollowed] "but ye were justified"[through the accepted righteousness of Christ-- these are his born again brethren whom he is threatening with exclusion from the Kingdom] "defiled, ye were cleansed; profane, ye were hallowed; unrighteous, ye were justified.  Dare any of you become foul again?" Paul askes.  Now if unbelievers only are to be excluded, then Paul's warning is not only pointless, but unjust.  Believers are sinning;  unbelievers are to be excluded: "ye do wrong"; therefore the world will be punished:  does God reveal the sins of one group of men, to threaten punishement to another? "I fear lest I should find you not such as I would," (because of) "uncleanness and fornication and lasciviousness which they COMMITED" (2 Cor. 12:20)
It is the washed, the sanctified, the justified that are in peril.  Are hypocrites - empty professors, false brethren, who have slipt pas the Church examiners - washed, sanctified, justified? Hear what the Spirit is saying to the churches:-- "HE THAT DOETH WRONG SHALL RECEIVE FOR THE WRONG THAT HE HATH DONE; AND THERE IS NO RESPECT OF PERSONS" (Col. 3:25).  "Thou hast a few names in Sardis which did not defile their garments: and THEY shall walk with me in white; FOR THEY ARE WORTHY" (Rev. 3:4).  (Who is worthy? Those few in Sardis which did not defile their garments!) "NOT EVERY ONE THAT SAITH UNTO ME, LORD, LORD"(something that the disciples uttered) "SHALL ENTER INTO THE KINGDOM OF HEAVEN but he that DOETH THE WILL of my Father which is in heaven" (Matt. 7:21). 

For it is exceedingly remarkable that in the very heart of the great Grace chapter of the Bible, the truth that a Christian's reward is exclusively determined by his own fidelity lies deeply embedded.  So then, it is by grace that we are saved.  Saved by grace, bought by the blood of Christ, and thereby justified and made eternally secure in eternal Heaven. However it is by righteousness and Godly living that may earn the reward of entrance into the 1000 year Milennial Kindom of God. 

Rev 20:6
6 Blessed and holy is he that hath part in the first resurrection: on such the second death hath no power, but they shall be priests of God and of Christ, and shall reign with him a thousand years.