First off Kerrylou. Welcome to the CU forums. And from reading your post I look forward to more discussions in the future.
And let me say also Amen to the rest of your post.
As to your reply to me, let me address a couple of points.
are you saying that there were no saved Hebrews?
Quite honestly I would say that there ARE no saved Jews. If they are Jewish then they are not Christian and thus not saved.

Now you started off looking at Hebrews 3:1. But lets get things in context and take a look even further back at Hebrews 1 and 2.
Hebrews 1:1 "God, who at sundrey times and in divers manners spake in time past unto the fathers by the prophets"
This verse lets us know that we are speaking to a group of people of Jewish decent. (and I do not think we are in disagreement that the epistle was directed to Hebrews/Jews).
Now the bulk of Hebrews 1 is setting up how much bigger Jesus is than anything ever brought forth before. Jesus is "heir of all", "the brightness of his glory". We are shown all the things that the Hebrews know of God, and were told of the Messiah.
Then in Chapter 2 the author states "we ought to give the more earnest heed to the things which we have heard..." The author equates himself with the readers. Granted this could be either as bretheren in Christ or as considering Jewish heritage. But the things we have heard is referencing the old teachings and ideals that were talked about in the previous chapter. So that narrows it down to talking about the Jewish aspect of things.
And the author continues to talk about how the Jewish faith has been shown all along what is to be Christ. "For if the word spoken by angels was stedfast", "which at the first began to be spoken by the Lord", "bearing them witness, both with signs and wonders",
Look down at Hebrews 2:11 "For both he that sanctifieth and they who are sanctified are all of one: for which cause He is not ashamed to call them brethren." Notice the perspective is not one of inclusion. The reader is placed outside looking in at Jesus and whome He calls brethren.
Further down is an appeal to the Hebrew family importance when the author mentions in verse 16 that Jesus was of the seed of Abraham. This is not important to Christians of any sort. But is dramatically important to the Hebrew faith where family heritage is vital.
Now we get to Chapter 3. In verse 1 take a look as you had pointed out.
"Wherefore, holy brethren, partakers of the heavenly calling"
There is a couple of things that stand out in this verse. First is the adjectives used such as "holy", the author seperates out these brethren from the use anywhere else. The priests considered themselves holy, the practicing jews considered themselves holy. Even as he continue with the things that would be important to Hebrew people. Even with OT references. And references to Moses and the time spent in the wilderness.
Now the Hebrews were in the living God, before Jesus came. Look at verse 3:12 "Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God" And go down to verses 14-15"For we are made partakers of Christ, if we hold the beginning of our confidence stedfast unto the end; While it is said, To day if ye will hear his voice, harden not your hearts, as in the provocation" And more references to the time of 40 years in the wilderness. Until ending in verse 19 "So we see that they could not enter in because of unbelief"
Which wraps up chapters 1 through 3 in which the reader was talking of how the Jews were the holy people and they were with the living God. And that they were God's chosen people. They were the heirs to salvation in that it was their job to pass along the word of God to the world. But we see that in the end if they do not believe in Jesus, as the author pointed out they have to listen to what God has been saying all along, then they will be like those that failed to believe in the wilderness and they will be "whose carcases fell in the wilderness"
I think that when clearly put in the context of the rest of the epistle and then with the rest of the Bible itself it becomes quite clear who the target of the epistle was unsaved Jewish people. And that when put into that light the verse used by the "losing salvation" crowd become invalid.
I hope that helps out in explaining where I come from as far as looking at Hebrews.
Again welcome to the forum