Every culture has sayings that become maxims or even mantras. "When the going gets tough, the tough get going." "A penny saved is a penny earned."
In the pastoral letters, Paul shares what appear to have been common sayings in the early church that he found "trustworthy."
Have nothing to do with irreverent, silly myths. Rather train yourself for godliness; for while bodily training is of some value, godliness is of value in every way, as it holds promise for the present life and also for the life to come. The saying is trustworthy and deserving of full acceptance. For to this end we toil and strive, because we have our hope set on the living God, who is the Savior of all people, especially of those who believe. (1 Tim 4:7-10 ESV)Paul didn't follow his usual format here, so it's hard to tell exactly where the "trustworthy saying" begins, but my money's on "train yourself for godliness." But it's instructive to see what he's contrasting that to. "Have nothing to do with irreverent, silly myths." Don't spend time spreading them, discussing them, or even fighting them. Spend your energy training yourself for godliness, because godliness — unlike physical fitness — has everlasting value.
Godliness doesn't just happen. It takes work. It takes discipline.
Do you not know that in a race all the runners run, but only one gets the prize? Run in such a way as to get the prize. Everyone who competes in the games goes into strict training. They do it to get a crown that will not last, but we do it to get a crown that will last forever. Therefore I do not run like someone running aimlessly; I do not fight like a boxer beating the air. No, I strike a blow to my body and make it my slave so that after I have preached to others, I myself will not be disqualified for the prize. (1 Cor 9:24-27).If you want to win a marathon, you're going to do things you don't want to do and skip things you want. Your goal will change your behavior, the choices you make, how you spend your time.
Godliness is no different. It requires daily deciding to kill the flash. It requires daily deciding to follow Christ, to walk in step with the Spirit. It's not for nothing that Jesus said to "take up your cross" and follow him.
Why put yourself through all that? We do what it take because we have hope "set on the living God" in Christ Jesus. He has promised that what we give up here will be more than made up later: "godliness ... holds promise for the present life and also for the life to come" when we will see “what no eye has seen, what no ear has heard, and what no human mind has conceived."
Our goal is worth the effort. We should strive to be like Christ because we will be with Christ.
So "let us run with endurance the race that is set before us, looking to Jesus, the founder and perfecter of our faith, who for the joy that was set before him endured the cross, despising the shame, and is seated at the right hand of the throne of God" (Heb 12:1-2).