Hebrews 3:18

And to whom sware he that they should not enter into his rest, but to them that were disobedient?

The Book of Hebrews makes a great thing of obedience, affirming that even Christ was made perfect by it (Hebrews 5:8,9) and that the salvation he authored is "unto all them that obey him"; and also marking especially the obedience of so illustrious a person as Abraham (Hebrews 11:8). In this verse, disobedience is made the basis of God's denying Israel the right to enter Canaan, the "rest" spoken of being a reference to their dwelling in that good land, rather than a mention of the sabbath day, the sabbath day, of course, being a rest that they did actually receive and enjoy throughout their whole history. In spite of the fact that the KJV translates this verse "believed not" instead of "disobedient," the English Revised Version (1885) is far preferable. Unbelief is indeed a sin, damning and destructive enough; but it is followed by overt and willful actions against the laws of God, such actions being of themselves fatal to the receiving of God's approval, no matter if founded in unbelief, as Israel's were, or not. One of the great heresies of the Reformation appeared in the doctrine of salvation by "faith alone" and the attendant notion that the only sin, actually, is unbelief. See more on this under Hebrews 11:6.