To properly understand what the author is saying, consider that in the last few verses of chapter 5, he departs from talking about the High Priesthood of Jesus to explain that some people are not maturing in their walk with God enough to handle such information. They have been saved long enough to be teaching others, but they still required that someone taught them again being unskillful in the word of righteousness (Hebrews 5:12). These set of people were moving back to Judaism because of the persecution that they were suffering from their Jewish brethren. It was a case of mixing self-righteousness and God’s righteousness, which is the recipe of confusion.
So Chapter 6 is Paul’s attempt to bring them back to the foundation of the New Covenant. He opens by taking them past the “principles of the doctrine of Christ”. He is not insinuating that there is something other to preach than Christ’s finished work, for the phrase means the “word of the beginning of the Christ” in Greek. He is taking them past the Old Testament prophets, which is where many Jews were getting stuck, still looking for Christ, though He had already come.
He wanted them to understand their perfection in Christ in verse 2, which he will further explained in the 10th chapter when he encouraged them avoid laying the “foundation of repentance from dead works”, this is the sacrificial system, and “of faith toward God”. Our faith is not toward God under the New Covenant, it is toward Jesus. That is why the emphasis is about Jesus and His faith at work in us.
So in this controversial place he said that “it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost, and have tasted the good word of God, and the powers of the world to come, if they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame” (Hebrews 6:4-6).
This place says nothing about loosing one’s salvation after growing through a complicated process and committing the unpardonable sin. What it mentions is that “to renew them again unto repentance”
Don’t fear losing your salvation today and don’t let that be your preoccupation. Continually open yourself to His unconditional love in your life daily, accept His forgiveness freely and allow Him to wash you with the water of His words. If you reject grace, there is nothing left to accept but your own works of the law for righteousness which cannot produce life in you. After being exposed to the grace of God in Christ Jesus resist every temptation to go back to your own works. Going back is to crucify Him afresh, saying that His finished work is an unfinished work.