Hi Brother Jon,
The "one another" in Hebrews 10:24 that we are to "provoke unto love" I presume you are using to support sharing the gospel with the unregenerate is in fact undeniably referring to fellow brothers in Christ, not undegenerates, when looked at in context. Here is the preceding verse you left out, "23 Let us hold fast
the profession of our faith without wavering; (for he is faithful that promised

" (Hebrews 10:23) The "us" in this preceding verse shows it is those who have already made a "profession...of faith", is this the unregenerate? Here is verse that follows verse 24 that you also left out by pulling the verse out of context, "25 Not forsaking the assembling of ourselves together, as the manner of some is; but exhorting one another: and so much the more, as ye see the day approaching.". Brother Jon, is verse 25 talking about the assembly of the brethren or the unregenerate?
One who has their sins atoned for is not lost. The word "atonement" is found only one time in the King James New Testament and it is here, "And not only so, but we also joy in God through our Lord Jesus Christ, by whom we have now received the atonement." (Romans 5:11) Notice those who "received" it received it through none other than Jesus Christ. This occurred 2,000 years ago on the cross. It does not say they "received" it through themselves upon hearing the gospel preacher, but by none other than Jesus Christ himself. I do not know much, but I know "our Lord Jesus Christ" by whom we "receive" the atonement, is not me, you, or the gospel preacher, as there is only one Jesus Christ. He either atoned for ones sin on the cross and they end up in heaven or he didn't and they go to eternal damnation.
Also, notice the Greek word used for "atonement" used in Romans 5:11) passage means "reconciliation" or "restoration to favor" per Strong's. (See
http://biblehub.com/greek/2643.htm ). Those whom God has made peace ("reconciliation") with and are "restored to favor" cannot by logic be considered "lost". The doctrine of an "atoned" lost person to share the gospel with is an oxymoron term!
Finally, when did this reconciliation take place according to scripture? Was it upon the individual believing the gospel? The preceding verse mentioning the atonement tells us, "For if,
when we were enemies, we were reconciled to God by the death of his Son, much more, being reconciled, we shall be saved by his life." (Romans 5:9). Notice the reconciliation occurred when we were "enemies of God" and it was "by the death of his Son", not at the time the gospel preacher arrived on the scene or by faith in the gospel, but "by the dead of his Son".
Here is a good in depth article on the doctrine of "atonement" I would recommend it to anyone who has the time and are interested in studying further by Elder Samuel Trott.
http://www.asweetsavor.info/est/atonement.php
God bless,
Brother Joe