> What do you believe happened when Peter denied Christ 3 times?
I think Pete was scared and weak. He wasn't denying his beliefs and convictions, nor his commitment to Jesus. He was acting in fear to save his own skin. Jesus restored him with a 3-fold expression in John 21.15-19. He didn't lose his salvation in a moment of weakness and failure, and neither do we.
> how do you interpret Hebrews 6:4-6?
It is often wondered whether the first half of Hebrews 6 implies (or says explicitly) that a Christian can lose his or her salvation. There are no less than seven places in Hebrews, however, where the author affirms that salvation is permanent:
1. Heb. 6.19: We have this hope as an anchor for the soul, firm and secure
2. Heb. 7.25: Therefore he is able to save completely those who come to God through him, because he always lives to intercede for them.
3. Heb. 9. 15: For this reason Christ is the mediator of a new covenant, that those who are called today receive the promised eternal inheritance—now that he has died as a ransom to set them free from the sins committed under the first covenant.
4. Heb. 10.14: Because by one sacrifice he has made perfect forever those who are being made holy.
5. Heb. 10.22: let us draw near to God with a sincere heart in full assurance of faith, having our hearts sprinkled to cleanse us from a guilty conscience and having our bodies washed with pure water.
6. Heb. 12.23: To the church of the firstborn, whose names are written in heaven...
7. Heb. 12.28: Therefore, since we are receiving a kingdom that cannot be shaken, let us be thankful...
In the book of Hebrews, the author assumes that no true Christian would ever fall away, and the only people who ever fall away were not true believers to begin with. He also assumes that just because you made a decision for Christ at one point in time is no indication that you are truly saved (see also the Parable of the Sower told by Jesus where the plants in soils 2 & 3 had accepted the Word but without true life and growth).
There are other powerful spiritual forces at work in people that can imitate and simulate the power of God (Mt. 7.22)—specifically designed and executed to deceive people (Mt. 24.24; 2 Cor. 11.13-14; Rev. 13.14). Neither having seen, tasted, and shared is ever a guarantee that a person is a true follower of Jesus. People use God for their own ends. People follow falsely and claim many things falsely. This strikes a common biblical note, namely how close to spiritual reality one may come while knowing nothing of its fundamental reality (e.g., Balaam; Judas Iscariot; Mk. 9.38-39; 1 Cor. 13.2; Heb. 3.14; 1 Jn. 2.19). 2 Timothy 3.5 speaks of "having a form of godliness but denying its power."
I argue that these verses (Heb. 6.4-6) aren’t a reference to Christians. See the context: 3.7-11; 4.2; 6.7-8. He is talking about the Jews who think they have an automatic "in" because they are Jews. But the author is saying that the Israelites in the wilderness weren’t spared God’s judgment just because they were Jews. No one gets a free pass. That also applies to us as Christians: You have to claim Christ and walk in Him, not just be along for the ride.
There's plenty more to talk about in these verses, so ask if you want to pursue it further.