Hebrews 4:1-2

Therefore, let us fear if, while a promise remains of entering His rest, any one of you may seem to have come short of it. For indeed we have had good news preached to us, just as they also; but the word they heard did not profit them, because it was not united by faith in those who heard.

v 1 - "Although God has promised believers today that they may enter His rest, some may fail to experience it because of unbelief." (Ryrie)

"A promise being left" (v. 1).  "There remaineth therefore" (v. 9), i.e., He still leaves a promise, and there still remains a rest.  This Gospel rest, and the kingdom connected with it, "remained" from Pentecost to the destruction of Jerusalem.  It was nationally rejected through unbelief, and lost.  But just as the children (Neh. 9) entered Canaan, so will the future children be engraced with the faith of their believing fathers (Mal. 4:6) and will look upon Him with faith who loved them and washed them from their sins in His own blood (Rev. 1:5).  The glad tiding preached in the Wilderness promised the milk and honey of Canaan; the glad tidings preached during the Pentecostal era announced forgiveness of sins and the gift of the Holy Spirit (v. 2)." (Williams)

v 2 - "To understand this verse we must identify the pronouns.  "Us" refers to the first-century Jewish readers of this letter, "them" to the generation which came out of Egypt.  The words "the gospel was preached" are the translation of a verb which means "to announce good news."  The character of the good news must be defined by the context.  The good news which was announced to the first-century readers of this epistle was that of a spiritual rest in Messiah.  The good news given to the generation which came out of Egypt was that of a temporal, physical rest in a land flowing with milk and honey, offered to a people who had been reduced to abject slavery for 400 years and who had lived on a diet of leeks, garlic, and onions during that time.  But the writer says that this good  news did not profit this generation, "not being mixed with faith in them that heard it."" (Wuest)

mixed with faith - "This does not mean that the glad tidings were profitless to them because they did not add faith as they listened.  It is most true that salvation results from listening and believing, but the argument of this verse is that there were two companies, the company of the believers and the company of the unbelievers, and because these latter were not one in faith with the former, therefore, they failed to enter Canaan." (Williams)