John Chrysostom
Here "Today" means "always." What do these words mean? Since someone must certainly enter, he says, and those men did not enter, God appoints another, third rest. Let us hear how he proves that someone must enter, and that someone certainly will enter. He proves it by the fact that "after so long a time" David again says, "Today, if you hear His voice, do not harden your hearts." Of which hardened men does he remind them, and of which unbelievers? Is it not the Jews? The meaning of his words is this: they heard, he says, just as we hear, but they received no benefit. Therefore do not think that you will benefit from the mere hearing of the preaching. They also heard, but received no benefit, because they did not believe.
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But Caleb and Joshua, who were not mixed with the unbelievers, that is, were not of one mind with them, escaped the punishment that befell them. And this is remarkable: it does not say that they did not agree, but that they were not mixed. That is, they did not take part in the uprising, while all the others rose up with one accord. Here, it seems to me, the Apostle is hinting at some disturbance.