Who are those who have once been enlightened and have shared in the Holy Spirit?

[tonychanyt]

Is it impossible for those fallen away to come back into repentance?

Jesus speaks in Matthew 13:

7 "Other seeds fell among thorns, and the thorns grew up and choked them. 8 Other seeds fell on good soil and produced grain, some a hundredfold, some sixty, some thirty."

The book of Hebrews describes a similar idea in 6:

1 Therefore let us leave the elementary doctrine of Christ and go on to maturity,

The author contrasts elementary doctrine vs maturity. The elementary doctrine includes:

not laying again a foundation of repentance from dead works and of faith toward God, 2 and of instruction about washings, the laying on of hands, the resurrection of the dead, and eternal judgment.

But we need to grow to reach maturity.

3 And this we will do if God permits.

Now comes a warning:

4 For it is impossible, in the case of those who have once been enlightened, who have tasted the heavenly gift, and have shared in the Holy Spirit, 5 and have tasted the goodness of the word of God and the powers of the age to come,

The above describes experienced Christians who have been born in the Spirit. They stay like infantile newborns for years. They are like seeds fell among thorns (Matt 13:7).

6 and then have fallen away,

That's bad, very bad. They are not growing in the Spirit and have stopped believing.

to restore them again to repentance, since they are crucifying once again the Son of God to their own harm and holding him up to contempt.

That's a strong warning against those long-time childish Christians. However, what is impossible with men is possible with God (Matt 19:26).

Now comes a blessing for the experienced mature believers:

7 For land that has drunk the rain that often falls on it, and produces a crop useful to those for whose sake it is cultivated, receives a blessing from God.

Mature believers are like seeds that fell on good soil (Matt 13:8).

But there is a curse for the experienced immature Christians:

8 But if it bears thorns and thistles, it is worthless and near to being cursed, and its end is to be burned.

The author didn't want to end this topic on a negative note:

9 Though we speak in this way, yet in your case, beloved, we feel sure of better things—things that belong to salvation. 10 For God is not unjust so as to overlook your work and the love that you have shown for his name in serving the saints, as you still do.

That's encouraging.

11 And we desire each one of you to show the same earnestness to have the full assurance of hope until the end,

Don't focus on the curse and the burning; Focus on the blessed hope.

12 so that you may not be sluggish, but imitators of those who through faith and patience inherit the promises.

Don't be sluggish. Go on ahead to grow spiritually to maturity.

Hebrews 6:1-12 contrasts between two kinds of Christians. There is a curse for the immature and a blessing for the mature believers. Don't stay at the elementary state. Press ahead to produce the fruit of a mature believer.

Is it impossible for those fallen away to come back into repentance?

It is possible for those who would repent again. It is impossible for those who stubbornly refuse to repent. Be warned.

See also Can we lose our salvation?.