In what did their inability consist? They were always going to need the help of God to enter into the promised land, for only he could defeat enemies who were physically stronger than they were. What they lacked therefore was not a natural ability.
What is faith? Faith is a small thing in itself, not great enough to warrant the infinite response of God’s grace and mercy. It can be seen as the absence of something: the absence of unbelief, the refusal to credit God with integrity, to see him as unworthy of our trust. It is such a reasonable thing to trust God, because of our created state of dependence on him, that to refuse to do so is highly provocative. It is to insult him in the matter in which he most deserves to be trusted: his words. God cannot lie and the words that come from him should be treated as life giving droplets that come out of a well of life, as more precious than gold. But unbelief spurns his kindness and insults him, and despises his gift which we do not deserve. When God speaks to us words of kindness even in our sinful state, it is such an undeserved overture on his part that we should give thanks and worship him that he has even deigned to speak at all. No wonder God withdrew his blessing from ancient Israel, and did not even give them the typical blessings which he had held out to them. God makes a point of judging that generation so that it would be crystal clear for ever, that faith is the condition of blessing.