Heb 3:19

19 So we see that they could not enter in because of unbelief.

NKJV

It would seem that unbelief is an impediment to entering His rest. Doesn't this beg the question, "What exactly is unbelief?"

Examining the passage Hebrews refers to, we're talking about the Hebrews in the desert, all of whom, but two, did not believe God. All of whom but two.

In what way did they not believe God? In that, after they returned from spying out the promised land, decided that the promise was too difficult to keep. Though Joshua and Caleb named the promise, and claimed the victory, the entire nation sided with the seeing of the eyes and the hearing of the ears and turned away from God...following the teaching of their elders.

And now we're exhorted by the author not to follow after their example of unbelief. It's a dilemma, isn't it?

On the one hand, we have Jesus who said, "Ask whatsoever you will, and it will be done unto you." Or, "Whatsoever you ask the Father in My name, it shall be done..." Or, "...and doesn't doubt in his heart, he shall have whatever he says." And then you have the orthodoxy, who stand in the doorway to faith with an entire list of reasons why God will not do as Jesus said He will, beginning with the idea, that whatever you ask must be perfectly alligned with his "perfect" will (a whole doctrine in itself), and if its not, there is no reason you should expect God to do anything according to your prayers. By their doctrine, nothing is done but by the will of God, and that will is absolutely inscrutable. Though "knowing God" is what they extol, they teach a God who absolutely cannot be known.

On the one hand you have the commandment to come boldly, and make your requests known to God with thanksgiving. And on the other hand, you have the orthodoxy, who decry an attitude that is anything but grovelling. "I am not worthy so much as to gather up the crumbs under thy table."

Theirs is the way of blind Bar Timideus, who, upon hearing that Jesus was passing by, shouted once, "Jesus, Son of David, have mercy on me...", and was instantly set upon by the entire orthodox church for his importunity. Believing that the elders knew more than he, he stayed quiet, hoping the sovereign Lord would look his way.

You don't know the rest of the story, because for some reason it did not end up in the testimony of scripture. Here it is: He stayed blind, and fulfilled the doctrine of the orthodoxy to the letter.