Home Sermons The Arithmetic of a Wise Heart
November 3rd, 2026
Today we look at a very unique psalm: Psalm 90. It is unique because it is the only psalm attributed to Moses. It connects with him at the end of his life, 120 years old, standing on Mt Pisgah and looking towards the Promised Land.
This Psalm starts and ends with God, the sovereign and eternal God who is the only answer for us humans, whose lives can be summed up by the psalmist in two words: BRIEF and GRIEF.
The length of our days in seventy years, or eighty, if we have the strength. Psalm 90:10
Not much has changed in the thirty-four centuries since Moses wrote those words. Statistics NZ tell us that the average life expectancy for men is 80, and for women 84 years. Mind you, I think that we could now change it from 80 to 85.
Rick Warren, in his book The Purpose Driven Life, says this:
One of the biggest ways you can waste your life is thinking all there is, is here and now. You’re going to spend far more time on that side of death, than you do on this side. On this side, you may get to live 60, 70, 80 or 90 years or so. But that’s just a drop in the bucket compared to eternity. The time you spend on earth really isn’t that very long. It’s like if you stretched out a line from Tokyo to Los Angeles, across the Pacific Ocean. The first millimeter of that line would not even equal how tiny your life is compared to all the time you’re going to spend in eternity. You were made for eternity and life is preparation for eternity. Rick Warren, The Purpose Driven Life
What Moses gets us to do in this psalm is to have a good look at how brief our life is, and how we best deal with that. He gives some images to show us the brevity of life:
You may have heard the story of the little girl who learned in Sunday school that man came from the dust and eventually returns to the dust. She looked under her bed one morning and said, “Mother, mother, come! There’s someone under my bed, but I don’t know whether he’s coming or going!” That is a child’s way of underscoring this great truth. God sets limits to life. We come from the dust and we return to the dust.
Now, Moses in this psalm seems to get quite morbid. He is preoccupied with time. In fact units of time (morning, evening, night, days, years) are referred to 15 times in these verses. Moses reminds us that for each of our lives the clock is ticking; the hour glass of life is running down!
To make it worse, Moses also makes a number of statements about our lives being full of trouble and grief.
The length of our days is seventy years, or eighty, if we have the strength; yet their span is but trouble and sorrow, for they quickly pass, and we fly away. Psalm 90:10
Life has Grief: Our lives are shot through with experiences of trouble, sorrow, and disappointment. Now, in one way we can accuse Moses (and me, if you like, since I am dwelling on all this) of being depressing. But in another sense you can argue that we are being realistic.
Moses, in the light of life being brief and splattered with grief, approaches God with a prayer. This is how the NIV puts verse 12:
Teach us to number our days aright, that we may gain a heart of wisdom. Psalm 90:12 NIV
Teach us to make the most of our time, so that we may grow in wisdom. New Living Translation
The New Testament equivalent is provided by the apostle Paul in Ephesians 5:15–16:
Be very careful, then, how you live—not as unwise but as wise, making the most of every opportunity, because the days are evil. Ephesians 5:15–16
This is the key challenge of the psalm. How do we gain a heart of wisdom? Or, how do we number our days aright? How do you and I do the arithmetic of a wise heart?
We do live for a finite time, most of us well into our old age, but some tragically are cut short early on. Others slip away after a period of illness. But the statistics indicate that 100% of people die!
It is only a relationship with the eternal God that gives both the brief and grief aspects of life some meaning, and then provides a genuine alternative.
Moses affirms that God is our constant help and our hope. He opens the psalm with this majestic affirmation, when he states that God is our home:
O Lord, you have always been our home. Psalm 90:1
And our opening hymn, taken from this psalm, had as its first verse:
O God, our help in ages past,
our hope for years to come,
our shelter from the stormy blast,
and our eternal home!
Psalm 90 refers to several features of God:
God’s eternity. The contrast and answer to our own brevity!
God’s wrath. This is a really uncomfortable reality. I wonder whether it is the reason that the lectionary reading actually omits verses 7 to 12! They refer to God’s anger and wrath five times! It all goes back to Genesis 3, after Adam and Eve disobeyed God’s command. God’s wrath, his righteous anger, is all connected to our sin and rebellion. God’s wrath is real! However, while God cannot condone our rebellion, because of his great love he cannot leave us stuck in it. The psalmist says:
Satisfy us in the morning with your unfailing love. Psalm 90:14
How do we cope with the reality of both God’s wrath and God’s love? Let me illustrate. Cancer surgery. God’s anger is directed against the cancer of sin in our lives and in our world. But his love caused Jesus to come and deal with it.
So the heart of wisdom engages with God and cries out: “Turn, O LORD! How long? Have compassion on your servants!”
As we engage with God we also engage with the third part of our reality check. The psalmist engages with several key aspects of life in the final verses of this psalm. I like the Message translation of verse 12:
Oh! Teach us to live well! Teach us to live wisely and well! Psalm 90:12 The Message
We count our blessings. As part of living well, I think the psalmist suggests we do another form of arithmetic. Moses engages with blessings that mean we live wisely and well:
Satisfy us in the morning with your unfailing love, that we may sing for joy and be glad all our days. Make us glad for as many days as you have afflicted us, for as many years as we have seen trouble. May your deeds be shown to your servants, your splendor to their children. May the favor of the Lord our God rest upon us; establish the work of our hands for us—yes, establish the work of our hands. Psalm 90:14–17 NIV
The blessings are those of gladness, involvement of our children, and success of our efforts. What I label laughter and legacy.
Do you want to live wisely and well? Then laugh a lot! Do you think God laughs? Where does our sense of humour come from? Decades ago people thought it was wrong to be light-hearted in church; deadly serious stuff. Now I do know that the Bible concept of joy is far deeper and stronger than happiness and humour, yet I know that humour is an important part of our humanity.
What about legacy? Moses suggests two strands: our family and our work.
The opening verse of this psalm:
Lord, you have been our dwelling place throughout all generations. Psalm 90:1 NIV
Then Moses prays for something much more specific:
May your deeds be shown to your servants, your splendour to their children. Psalm 90:16 NIV
This is an amazing request. We moderns are inclined to see life out of focus. We seldom think of ourselves as being part of a bundle of life which goes back to the very beginning, unless you are into researching your family tree. And there is something in human life that persists from one generation to another.
When you or I become a Christian and God begins to heal our personal life, we may never experience the full effect of that healing in our lifetime. But our children will! That is what the psalmist is saying here. Notice how he puts it: “May your deeds be shown to your servants,” i.e., his own generation. And then “your splendor to their children.”
Do you also want to feel that your work is significant? That is another kind of legacy.
May the favour of the Lord our God rest upon us; establish the work of our hands for us—yes, establish the work of our hands. Psalm 90:17
I guess that all of us want to leave our workplace in a better situation than when we entered it. But can I suggest, in this age when retirement is becoming a very fluid notion, that any work that we do, paid or unpaid, is significant. I love this quote from a book by Tom Wright:
What you do in the present—by painting, preaching, singing, sewing, praying, teaching, building hospitals, digging wells, campaigning for justice, writing poems, caring for the needy, loving your neighbor as yourself—will last into God’s future. These activities are not simply ways of making the present life a little less beastly, a little more bearable, until the day when we leave it behind altogether. They are part of what we may call building for God’s kingdom. N. T. Wright, Surprised by Hope (Harper Collins, 2008), p. 193
Teach us to number our days aright, that we may gain a heart of wisdom.
How do we do this? Through doing a reality check, one that embraces the realities of death, God, and life. And we engage with life through counting our blessings, embracing both laughter and legacy.
Earlier on I highlighted that this psalm, written by Moses, states that for us all:
Life is brief, and
life has grief.
But when we trust in God,
then we find relief.
