I think Jerry Bridges may be the one who made this somewhat popular, though my apologies if I am wrong. I have heard about it from him, by way of others as well.
Here are four of my own reflections on the need for preaching the gospel to yourself, particularly in the midst of suffering or confusion about what God is doing in your life.
By preaching the gospel to ourselves daily, we are reminding ourselves that Jesus died for sinners like me. We are reminding ourselves that our greatest problem is our sin and Jesus died to free us from that. Whatever problems we now have pale in comparison to our sin, and are more easily met through the cross.
By preaching the gospel to ourselves daily, we are reminding ourselves of the commitment that God made to us in Christ. “He who did not spare His own Son, but delivered Him over for us all, how will He not also with Him freely give us all things?” (Romans 8:32). The commitment of God is such that, having solved our biggest problem—the problem of sin, he will not leave us to be devoured by lesser problems. We are reminding ourselves that, having trusted God with our eternity, it would be foolish not to trust him with our present life.
By preaching the gospel to ourselves daily, we are reminding ourselves that depression, discouragement, confusion, and even death are not worthy to be compared with the eternal weight of glory that God will reveal in us at his second coming (2 Cor 4:17 ; Rom 8:18). By preaching the gospel to ourselves daily, we are reminding ourselves that “it’s only for a little while.” Whatever “it” is, it will not least forever, even if it never goes away in this life.
By preaching the gospel to ourselves, we are reminding ourselves that God has much more at stake in it than we do. We have only one life to manage. If we mess it up, it is a relatively small mess. However, God is managing the entire universe. If God lets us down, we have lost things like my comfort, my ease, my happiness, my family, or perhaps my life. But God has lost everything—his universe, his promises, his trustworthiness, and his character, in fact, his very existence, since God cannot cease to be what he is without ceasing to be. He who has the greatest stake will make the greatest effort to ensure the outcome.
Personally, I find Romans 8:26-38 an incredible way of reminding myself of the gospel and how much I need it, even after more than three decades of having it.