If you look again you will see μετόχους used here and may get the sense of what "partaker" means.
Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God. But exhort one another daily, while it is called To day; lest any of you be hardened through the deceitfulness of sin. For we are made partakers of Christ, if we hold the beginning of our confidence stedfast unto the end;
That doesnt change a thing. Its rejecting the Holy Spirit that leads to no chance for redemption. The Jews have rejected Christ but will one day be redeemed. Its not Christ that those in Hebrews 6 rejected but it is the Holy Spirit. One has to keep in mind that its through the Holy Spirit that now works that we are called. Rejecting that Holy Spirit is what is being talked about in Hebrews 6. Not the rejection of Christ. Rejecting the Holy Spirit leaves one with no one to work in them for redemption. Nice try though.
Those who believe, receiving Jesus, are those who are sealed with the Holy Spirit until the day of redemption through that same believing that gave them the right, or authority, to become the children of God.
Believe - Greek
http://biblehub.com/text/john/1-12.htm
John 1:12 But to all who did receive him, who believed in his name, he gave the right to become children of God,
Strong's Concordance
believe - pisteuó: to believe, entrust
Original Word: πιστεύω Part of Speech: Verb Transliteration: pisteuó Phonetic Spelling: (pist-yoo'-o) Short Definition: I believe, have faith in Definition: I believe, have faith in, trust in; pass: I am entrusted with.
HELPS Word-studies
4100 pisteúō (from 4102 /pístis, "faith," derived from 3982 /peíthō, "persuade, be persuaded")
believe (affirm, have confidence); used of persuading oneself (= human believing) and with the sacred significance of being persuaded by the Lord (= faith-believing).
Only the context indicates whether 4100 /pisteúō ("believe") is self-serving (without sacred meaning), or the believing that leads to/proceeds from God's inbirthing of faith.
http://biblehub.com/ephesians/1-13.htm
Ephesians 1:13 In him you also, when you heard the word of truth, the gospel of your salvation, and believed in him, were sealed with the promised Holy Spirit,
In Greek: http://biblehub.com/text/ephesians/1-13.htm http://biblehub.com/greek/4100.htm