Forgetfulness comes naturally to most of us. But it is highly selective.

My neighbor Sandy told me about an acquaintance who left the grocery store, went on another errand, then realized she had forgotten her groceries. She went back and checked all the carts in front of the grocery. Her bags were not there. Someone must have taken them.

She marched in to tell the store manager about the crime. "Could you describe your bags, ma'am?" he asked. "Paper or plastic?"

"They were plastic," she said confidently. "There were two." She went on to carefully list their contents.

"Oh," said the manager. "And might they be the two plastic bags that are hanging from your arm?"

The woman looked down. There they were, dangling from her left elbow.

Another friend—I'll call her Jan—was traveling to Florida with her family one Easter break. She and her husband were taking turns driving. Their kids had on their pajamas; they were cozily snoozing. Jan's husband took the first shift, while she rested. At about midnight they traded, and he wearily crawled to the back of the van to get some sleep.

At 1 o'clock in the morning Jan was getting dangerously drowsy, so she stopped at an all-night place to get coffee. While she was paying for it, her husband came in the store, signaling to her that he was going to use the men's room.

Jan got back into the van and sipped her coffee. Sure enough, it perked her up. She was able to drive the next five hours without a problem. As the sun rose and the car was crossing the Florida border, the children yawned and stretched awake.

"Guys," said Jan, "can you wake Daddy up, please? We're going to stop for breakfast."

The oldest child unbuckled, turned, and leaned over into the back of the van.

"Mom?" he said. "Uh—Dad's not here."

Yes. Jan, who had forgotten to turn her cell phone on, had also forgotten her husband at the coffee shop, which was now 300 miles away.

Most of us have stories like these. Forgetting seems to come quite naturally.

But why is it that we forget the wrong things? Why is it so easy to forget the spiritual truths we need to remember, like God's incredible love for us, His gracious work in our lives, His Word?

And conversely, why is it so easy to remember what we'd just as soon forget?

For example, I retain in my brain not just part but all of the lyrics to the theme songs to the most innocuous television shows of the 1960s and '70s. On a moment's notice, I could easily burst into all the stanzas of The Beverly Hillbillies, Gilligan's Island, and, let's see, Green Acres. And the sad fact is, if you're old enough to remember this golden period of television, you could sing right along with me.

Why do I have total retention of Gilligan but not of Scripture? Why do many of us remember in great detail every embarrassing thing we've ever done? Why are some of us haunted by sins from our past, carrying their shame as if it was yesterday?

Remembering God's faithfulness is a key part of our spiritual development. But, at the same time, before our spiritual vitality can really flow, we must forget three things.

A friend told me about his son's challenges with his car, and consequently with his auto insurance. It seemed that the son had incurred some reckless driving offenses that were an indelible part of his record. Accordingly, his insurance costs were approximately astronomical.

My friend went through a number of procedures; his son went to driving school; a judge was gracious. And the record was expunged.

It was too good to be true. Just for fun, my friend called the dmv, cited his son's entire name and Social Security number, and asked about the citations on his record.

There was a long pause while the staff person checked the computer. She came back on the line. "Sir," she said, "we have no record of any offenses at all."

As my friend says, that's how forgiveness is. The fact that God forgets is like calling heaven and hearing, "There is no record of any offense." It's not that the offenses are still on the page, crossed out with heavy marker, and everyone can tell there was a big mess right there. Nor are they partially erased, but still visible. They are expunged. It is as if they never existed.

We need to take God at His word. The extent to which we dwell on and obsessively noodle on our own sins from the past—sins for which we have repented—shows the extent to which we really believe God.

Satan would lure us to live in guilt and fear. He pulls us into the shadow boxes of memory, in which our worst sins replay on the walls of our minds, flickering with their shameful power … what if we just can't forget our sins?

We need to be very clear here, or we'll miss the liberating point. It's not as if forgiveness is real only if we experience some kind of holy amnesia. It is not up to us. It is up to God. And the Scriptures say that God forgets our sins (Heb. 8:12).

Sometimes, even if we have mentally let go of various sins, their residual shame still sticks. Shame can spur depression, and dysfunction of every kind.

The apostle Paul could have been a prime candidate for shame, hopelessly glugging first-century wine in order to forget the horrors of his past sins. He writes of not just his pharisaical pride, but the fact that he had presided over the harassment, imprisonment, torture, and murder of innocent human beings.

When I am tempted to despair in shame over past personal choices, I have found Paul's "Popeye prayer" in 1 Corinthians 15:10 enormously helpful: "But by the grace of God I am what I am, and His grace to me was not without effect" (emphasis mine).

God's grace is stronger than our shame—most supremely, because it was defanged when Jesus died and rose again. We can choose to believe this or not. But God's magnificent provision is more than ample: we can forget our slithering shame, for Jesus took it. He scorned it, beat it, broke it. We need not take it back.

This doesn't necessarily mean that we must forget how we won the sixth-grade spelling bee, or the great projects we've completed, or viable accomplishments in various endeavors. We are to work and live with excellence as unto God, and He takes pleasure in our successes as they are offered to His glory.

But we should forget past successes if they have defined our identity or caused us to be complacent, smug, and self-congratulatory. We're not to fondle the past, dwelling on past glories. (Similarly, we must make sure that our stories of God's work and grace in our lives are not all old. If we can only point to instances of God's faithfulness from five years ago but have none from this week, then our connection with Christ is not very current.)

Again, Paul is a great model. In one of his letters he warns the Philippians against those who would put their confidence in human criteria and accomplishments.

If anyone else thinks he has reasons to put confidence in the flesh, I have more, he said, citing his power résumé that many of his day would have coveted. He could have been a total prig. But his focus was not on his past.

Again, Paul had learned that the secret of being content did not depend on what he had, but who he knew. He knew the Lord was near. He knew how to live so immersed in Him that gracious gratitude—and the corresponding peace of God—overwhelmed his earthly fears.

The only obstacles that keep us from the rich freedom that Paul enjoyed are our own cheap snares and doubts. We get lost in these when we look to ourselves, preoccupied by our sin, our shame, our success, as if it were all about us. But if we can forget these tangles and leave them behind, like Paul, we begin to see that God's love is bigger than we ever dared to dream.