What does Psalm 103:18 mean?

In this verse David identifies those who fear the Lord. This is not a cowering, terror-filled kind of fear. This is the humble respect given from a created being to their Creator. Those who "fear" God honor and obey Him. Within the nation of Israel, this meant those who kept the covenant the Lord made with Israel and who obeyed His commandments.

However, the Lord's promise to reward generations of Israelites was not an unconditional or one-sided promise. Rebellious Jews could not claim it simply because they were members of God's chosen nation by birth. When leadership was about to pass from Moses to Joshua, the Lord told Moses the people of Israel would eventually become idolatrous. He said, "Then my anger will be kindled against them in that day, and I will forsake them and hide my face from them, and they will be devoured. And many evils and troubles will come upon them, so that they will say in that day, 'Have not these evils come upon us because our God is not among us?'" (Deuteronomy 31:17).

The religious leaders in Jesus' day mistakenly relied on their physical link to Abraham, the father of the nation of Israel, to credit them with righteousness. In contrast, Jesus told them their actions proved they belonged to the Devil (John 8:39–44). Jesus taught that His friends are those who obey His commands (John 15:14).