Hebrews 4:8

For if Jesus had given them rest, then would he not afterward have spoken of another day.

Joshua's leading of Israel into Canaan did not give God's people the full and true rest He had promised, and the fact that Scripture still speaks afterward of another day proves that a further rest remains. If he had given the complete rest God intended, there would have been no later speaking of another day of rest. Israel's failure to reach the high ideal shows the need for another guide to bring the true rest.

God would have brought Israel at once and directly to the promised land if they had submitted to Him, trusted His leading, and loved to be led by Him. But their repeated rebellion and disobedience grieved Him many times and brought His oath in wrath that they would not enter His rest, so they wandered long in the wilderness. Only the two who wholly followed Him were excepted from that oath. He required His people to trust Him alone rather than seek help from those who did not serve Him.

A greater leader than Joshua, Jesus Christ, is in the midst of God's people to lead them to victory, which is the true rest of faith. All power in heaven and earth has been given to Him as the Captain and leader of salvation. They must submit to be led by Him and obey Him if the promise is to be fulfilled for them, and if they fail to obey, His promise cannot be fulfilled to them. The verse is part of a warning that unbelief can keep believers from entering God's rest, just as it kept the Israelites from entering their rest. Merely knowing God's truth is not enough to enter His rest if people do not follow what they know.

The rest still spoken of after Joshua is the heavenly Canaan, and God's people are now on its very borders. Entering that rest requires advancing with increasing light into purity and holiness. It requires faithfully improving the entrusted talents so they greatly increase, and faithfully accepting and valuing the light given. It requires taking a high and noble stand in self-denial and self-sacrifice to be channels of light to the world, for where much is given much is required. Those who let their precious condition grow dull through lifeless formality and do not advance go backward on the very borders and fail to enter.