In regard to Hebrews 6:4-6, once enlightened, which means to bring to light, to shed light upon or to cause light to shine upon some object, in the sense of illuminating it. John 1:9 describes Jesus, the "true Light," giving light "to every man," but this cannot mean the light of salvation, because not every man is saved. The light either leads to acceptance of Jesus Christ or produces condemnation in those who reject the light.
In regard to partakers of the Holy Spirit, the word translated “partaker” can certainly refer to a saving partaking in Christ, as we read in Hebrews 3:14, yet it can also refer to a less than saving association or participation. See Luke 5:7 and Hebrews 1:9 - "comrades, companions," which describes one who shares with someone else as an associate in an undertaking. These Hebrews who fell away had obviously in some aspect shared in the ministry of the Holy Spirit, but in what way? There are other ministries of the Holy Spirit which precede receiving the indwelling and sealing of the Holy Spirit, which only genuine believers receive.
Those who fall away absolutely could have been affiliated closely with the fellowship of the church. Such people certainly may have experienced sorrow for sin, heard and understood the gospel and have given some assent to it and have become associated with the work of the Holy Spirit while around believers and have tasted the heavenly gift and the powers of the age to come. They may have been exposed to the true preaching of the word of God yet have simply tasted and stopped there. People who have experienced these things may be genuine Christians, yet this alone is not enough to give conclusive evidence that the beginning stages of conversion (repentance unto life, regeneration, salvation, justification, etc..) have taken place for those who fell away. The experiences in Hebrews 6:4-6 are all preliminary to those decisive beginning stages of becoming a Christian, yet those who draw back to perdition after receiving the 'knowledge' of the truth do not believe to the saving of the soul. (Hebrews 10:39)
These certain individuals who fall short of obtaining salvation certainly may have become partakers of the Holy Spirit in his pre-salvation ministry, convicting of sin and righteousness and judgment to come by tasting the good word of God and temporarily responding to His drawing power which is intended to ultimately lead sinners to Christ, yet the writer of Hebrews does not use conclusive terms that these individuals were "indwelled by the Holy Spirit" or "sealed by the Holy Spirit." Genuine believers who have believed the gospel are sealed in Him with the Holy Spirit who is the guarantee of our inheritance until the redemption of the purchased possession/unto the day of redemption. (Ephesians 1:13-14; 4:30)
In regard to tasted the good word of God and the powers of the age to come, they may have tasted in such a way as to give them a distinct impression of what was tasted, yet they still fell away. Inherent in the idea of tasting is the fact that one might or might not decide to accept what is tasted. For example, the same Greek word (geuomai) is used in Matthew 27:34 to say that those crucifying Jesus "offered him wine to drink, mingled with gall; but when he tasted it, he would not drink it." We do not merely taste, but drink into one Spirit. (1 Corinthians 12:13)
In regard to renew them again unto repentance, this does not specify whether the repentance was merely outward or genuine accompanied by saving faith. They have in some sense "repented," there may be sorrow for sins and an attempt to turn from them (moral self-reformation) that non-believers can experience. There is repentance that falls short of salvation, which is clear from Hebrews 12:7 and the reference to Esau, as well as the repentance of Judas Iscariot in Matthew 27:3. Paul refers to a repentance “without regret that leads to salvation,” which shows there is a repentance that does not lead to salvation. As with “belief/faith”, so too with “repentance,” we must always distinguish between what is substantial and results in salvation and what is spurious. Renew them again "unto salvation" would be conclusive evidence for the argument of a loss of salvation.
In Hebrews 6:7-8, we read - For the earth which drinks in the rain that often comes upon it, and bears herbs useful for those by whom it is cultivated, receives blessing from God; but if it bears thorns and briars, it is rejected and near to being cursed, whose end is to be burned. In this metaphor relating to agriculture, those who receive final judgment are compared to land that bears no vegetation or useful fruit, but rather bears thorns and thistles. We see in scripture where good fruit is the evidence of spiritual life and a lack of good fruit is a sign of false believers (Matthew 3:8-10; 7:15-20; 12:33-35) so we have an indication that the trustworthy evidence of one's spiritual condition is the fruit they bear (whether good or bad), suggesting that those who fell away in Hebrews 6 were not genuine believers.
*Verse 9 sums it up for me. The writer is speaking to those truly saved (refers to them as BELOVED). He says that even though he speaks like this concerning THOSE types of people, He is convinced of better things concerning YOU. Things that ACCOMPANY SALVATION. Thorns and briars and falling away permanently do not accompany salvation and are not fruits worthy of authentic repentance.
It's generally stated by those who believe that salvation can be lost that it can be regained again, yet that would not be the case here if the writer of Hebrews was teaching a loss of salvation. I have heard certain individuals state they know someone who was truly saved, but later lost their salvation, yet only God truly knows the heart of individuals. Certain people "on the surface" may do a good job of looking like the real deal for a while (like Judas Iscariot, who was an unbelieving, unclean devil who betrayed Jesus - John 6:64-71; 13:10-11) yet to the other 11 disciples, he looked like the real deal, but Jesus knew his heart. There are genuine Christians and there are "nominal" Christians. There are genuine believers and there are make believers and you can't always tell them apart at first.
