John Calvin Commentary Joshua 15:63

"And as for the Jebusites, the inhabitants of Jerusalem, the children of Judah could not drive them out: but the Jebusites dwell with the children of Judah at Jerusalem unto this day." — Joshua 15:63 (ASV)

As for the Jebusites, etc. This provides no excuse for the people, nor is it recorded with that intention. For if they had exerted themselves to the full measure of their strength and failed to succeed, the dishonor would have fallen on God Himself, who had promised that He would continue with them as their leader until He gave them full and free possession of the land, and that He would send hornets to drive out the inhabitants.

Therefore, it was entirely due to their own sluggishness that they did not make themselves masters of the city of Jerusalem. They did not succeed in this; however, their own indolence, their neglect of the divine command due to a love of ease, were the real obstacles.

This passage deserves attention: we should learn from it to strive vigorously in attempting to accomplish the commands of God, and not to miss any opportunity, for fear that while we are idly resting, the door may be shut. A moderate delay might have been blameless, but a long period of self-indulgent ease effectively rejected the blessing God was ready to bestow.