This passage brings so pretty heavy things to heart: Let's look at it in sections. (This is me simply looking at the text w/o any study materials and drawing some basic conclusions."
Diving Deep: Grown-Up Teaching
Verses 1-3: Here the author of Hebrews, I take the stance that it was most likely Paul the Apostle, raises the idea that we should not go back to the basic fundamentals of the faith. Rather we should continue on to "grown-up teachings". Does this mean that we should stop teaching the fundamentals? At what point do we cease this teaching? When? I believe that here Paul is saying that there is more to God than simply the fundamentals. There is more to God than what we know. After we know and understand some of the basics we should dig deeper. We shouldn't just keep rehashing the same thing over and over again but rather we should dive deep into what God has for us. The kicker/reminder to all of this from Paul is that we can only go as deep as God allows. In verse 3 he says: "And we will go on to grown-up teaching if God allows."
Walking Out Of The Marvelous Light: Never To Return?
Verses 4-6: This verse hits hard for me. Paul is saying that there are those of us who have come to understand and accept the truth and life found in Jesus Christ and Him crucified and of those of us who believe this where there are those who "were once in God's light and enjoyed heaven's gift, and shared in the Holy Spirit." and then fell away or walked away from God. Paul quite plainly states that there are SOME of these people who cannot come back into God's light because by they walking away they are "nailing the Son of God to a cross again and are shaming him in front of others." I honestly don't have too much more to say on this beyond that I don't understand it all. How far does one walk away from God before it's too far? I've known friends who claimed to be followers of Christ only to walk away to complete Paganism only then to realize the error of their ways to turn back to Christ and now they follow Him whole heartedly. Were they not saved to begin with? The answer in my mind is not to see how close to the line we can tread but rather to turn away from the line and "press towards the prize".
Good Crop / Bad Crop
Verses 7-8: Here Paul is giving the comparison between the unbeliever and the Christ-follower. The believer who dives deep into the "grown-up teachings" and their relationship with God will produce good crops. They will be blessed and rewarded in their relationship with Christ for spending that time with Him. Those who don't know Christ and therefore also haven't spent time with him will dry up and due to their sin they will be "destroyed by fire." I believe verses 7-8 are a capstone ending and conclusion to the first two sections here. It addresses both sets of people as verse 7 addresses those who have been diving deep in the grown-up teaching while verse 8 addresses those who have either walked away from God or never accepted him in the first place.