The author of Hebrews writes, "Therefore, holy brethren, partakers of a heavenly calling, consider Jesus" (3:1) . . . "whose house we are, if we hold fast our confidence" (:6) . . . and it is, "Therefore, just as the Holy Spirit says" (:7). The author quotes, "'They tried Me, though they had seen My work for forty years'" (Ps 95:9-10). God said, "'Therefore I was angry'" (Heb 3:10) and that "'They shall not enter My rest'" (:11). Born-again Jews are the audience and the Spirit is the source, and logically the word "therefore" is accordingly used here three times. Is "rest" used as a type for Canaan representing heaven? Then how do you enter? With the New Covenant you experience salvation through repentance and confessing Jesus as Lord. But as "holy brethren" they were already saved having "entered" their salvation destination. "We who have believed enter that rest" (Heb 4:30).
Perhaps many of these Jews were becoming disenchanted and asking, as Israel did, "'Why is the Lord bringing us into this land, to fall by the sword?'" (Nu 14:3). They proposed, "'Let us appoint a leader and return to Egypt'" (:4). What was causing these doubts? God had already explained his intentions because that is why it is called the "promised land." The author subtly asks if they are actually listening to God by questioning "'If you hear His voice'" (Heb 3:7,:15,4:7). However, maybe they were aware of God speaking but had closed their ears. That could be why the author warns, "'Do not harden your hearts'" (3:8,:15,4:7) three times.