My sheep hear my voice, and I know them, and they follow me.
28I give them eternal life, and they will never perish, and
no one will snatch them out of my hand. 29My Father, who
has given them to me, is greater than all, and no one is able to
snatch them out of the Father's hand. - John 10:27-29 ESV
I will not leave you or forsake you. - Josh. 1:5 ESV
For the LORD will not forsake his people; - Ps. 94:14 ESV
The logical explanation of the verses in Hebrews is that those who fall away were never truly participators in the covenant to begin with. Indeed, reading the surrounding context in Hebrews is quite helpful:
So we see that they were unable to enter because of unbelief.
Therefore, while the promise of entering his rest still stands, let us
fear lest any of you should seem to have failed to reach it.
2For good news came to us just as to them, but the message
they heard did not benefit them, because they were not united by faith
with those who listened. 3For we who have believed enter
that rest... - Hebrews 3:19-4:3 ESV
As the author explains, those who did not enter never believed in the first place. This may seem strange considering what appeared to be a strong faith, so it's helpful to go to James on the subject.
What good is it, my brothers, if someone says he has faith but does
not have works? Can that faith save him? ...You believe that God is
one; you do well. Even the demons believe - and shudder! - James 2:14, 19 ESV
Typically faith is split into three categories: knowledge, assent, commitment. Most modern day Americans have knowledge (that is, most know of the gospel of Christ, or at least have heard of Jesus). Many believe that He died for their sins. But as James says, "even the demons believe!" Not so many, perhaps, actually trust God for their salvation. It is one thing to think that Jesus died for you, pray a prayer, and move on. It is another to live one's whole life utterly devoted to God, in light of Christ's death and wholly dependent upon that death for Salvation.
So, it seems the people in the church addressed in Hebrews were knowers, believers even. But "faith by itself, if it does not have works, is dead." (James 4:17). And dead faith is really no use to anyone. With a dead (nonexistent, i'd argue) faith, those people never really trusted in God in the first place - they were never Jesus's sheep in the first place, so they were not snatched out of his hand.
[from the perspective of a reformed presbyterian.]