But the mercy of the LORD is from everlasting to everlasting upon them that fear him, and his righteousness unto children’s children.
There is an old story… about the Jewish rabbi centuries ago who consented to take a weary traveler into his house for a night’s rest.
After they had eaten together, the rabbi said, “You are a very old man, are you not?”
“Yes,” the traveler replied, “I am almost a century old.”
As they talked, the rabbi brought up the matter of religion and asked the visitor about his faith and about his relation to God.
“Oh, I do not believe in God,” the aged man replied. “I am an atheist.”
The rabbi was infuriated. He arose and opened the door and ordered the man from his house.
“I cannot keep an atheist in my house overnight,” he reasoned….
But then the voice of God said, “Son, I have endured him for almost 100 years—don’t you think you could endure him for one night?” …
It was the mercy of God that had endured the atheist for nearly 100 years. ICH054-055
Lord, You have endured and continue to endure so much from Your creation. Your mercy is indeed from everlasting to everlasting. Amen. 
103:17, 18 With God’s mercy there is a vivid contrast. It lasts from everlasting to everlasting to those who fear Him. In duration, as in volume, it is limitless. And His righteousness extends to children’s children. There is great comfort in this. Christian parents often feel concern about their children and grandchildren growing up in a world of mounting wickedness. But we can safely entrust our little ones to One whose love is infinite and whose righteousness is sufficient not only for us but for succeeding generations as well. Of course, the promises necessarily have a condition attached. They are valid for those who keep His covenant and remember His commandments to do them. But that is only reasonable.
103:17, 18 the lovingkindness of the Lord. Those who appeal to God’s mercy by proper fear (v. 17) and obedience (v. 18) will overcome the shortness of physical life with eternal life. Lk 1:50 quotes Ps 103:17.
103:17–18 the steadfast love of the Lord is from everlasting to everlasting. Cf. 25:6; 100:5. Those who fear him (103:11, 13) are the same as those who keep his covenant and remember to do his commandments; they are the faithful, who believe the promises and obey the commands (Ex. 19:5; Deut. 7:9; cf. John 14:15, 21; 15:10; Rev. 1:3; 3:8). The covenant of circumcision, which Abraham’s descendants were to “keep,” included the promise that the Lord would be God to both the offspring and their parents. This psalm goes beyond that, however: the faithful expect that God sets his saving love on their children’s children. This is the crowning privilege that God gives to his faithful: though their lives are short and appear almost insignificant, they may still contribute to the future well-being of the people of God by their godly and prayerful parenting and grandparenting. Cf. also Ps. 100:5; 102:28; in Ex. 34:7a God keeps steadfast love for thousands (i.e., thousands of generations; cf. Deut. 7:9) for the faithful (Ex. 20:6).
103:17 everlasting to everlasting God’s loving nature has always characterized Him.
his righteousness The psalmist describes God’s righteousness (tsedaqah) as parallel to His chesed (“steadfast love”) for Israel. See v. 6 and note.
to their children’s children This multi-generational relationship between Yahweh and Israel is a feature of Yahweh’s covenant relationship with His people (see v. 18). Compare 90:16 and 102:28.
103:17 steadfast love … on those who fear him. There is a reciprocal relationship between divine initiative and human response. God first loves His people, then they love Him in return as shown in the faithful obedience of their lives (Rom. 5:8; 1 John 4:10–12).
 Tozer, A. W., & Eggert, R. (2015). Tozer on the almighty god: a 365-day devotional. Chicago, IL: Moody Publishers.
 MacDonald, W. (1995). Believer’s Bible Commentary: Old and New Testaments. (A. Farstad, Ed.) (p. 706). Nashville: Thomas Nelson.
 MacArthur, J. F., Jr. (2006). The MacArthur study Bible: New American Standard Bible. (Ps 103:17). Nashville, TN: Thomas Nelson Publishers.
 Crossway Bibles. (2008). The ESV Study Bible (p. 1069). Wheaton, IL: Crossway Bibles.
 Barry, J. D., Mangum, D., Brown, D. R., Heiser, M. S., Custis, M., Ritzema, E., … Bomar, D. (2012, 2016). Faithlife Study Bible (Ps 103:17). Bellingham, WA: Lexham Press.
 Sproul, R. C. (Ed.). (2015). The Reformation Study Bible: English Standard Version (2015 Edition) (p. 951). Orlando, FL: Reformation Trust.