Say what? Christ had already died, rose from the dead, and ascended into heaven already then?
Hebrews 3 is simply comparing the rebellion of the Israelites to rebellion in his day and the lack of faith. Hebrews 4 again is simply making a comparison. They both had good news of faith. Both are talking of the faith which was exhibited by obedience. Dispensation does not mean that the means of salvation has changed. I'm beginning to think you may need to understand the meaning of the word Paul uses in Ephesians 3:2.
Look in the mirror and repeat:
“I’m beginning to think you may need to understand the meaning of the word Paul uses in Ephesians 3:2.”
Seriously! Dispensation as used in the Bible means most closely “provision.”
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Hebrews 4:2 “For unto us was the gospel preached, as well as unto them”
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