I agree with Matt.
A passage that addresses this topic is 1 Tim 4:18 (from different translations):
(NET)
For “physical exercise [1] has some value, but godliness is valuable in every way. It holds promise for the present life and for the life to come.”
(NIV)
For physical training is of some value, but godliness has value for all things, holding promise for both the present life and the life to come.
(NASB)
for bodily discipline is only of little profit, but godliness is profitable for all things, since it holds promise for the present life and also for the life to come.
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NET's translation note [1] has:
Greek, "bodily training." NET also has the verse in quotations, indicating that this was probably (or possibly) a common saying among early Christians. NASB, the more literal of the three translations, gets to Paul's meaning the best, imo.
In the passage (context) Paul contrasts types of training: bodily training (which would have been understood as the rigorous training athletes underwent) - and - training in godliness. The analogy, and Paul's meaning, would be understood as "In comparison to what temporal benefits athletes derive from their rigorous physical training in this life - the eternal benefits of the exercise toward godliness are beyond compare!"
"Staying in shape" probably wasn't a problem for most century people. Paul worked as a tent-maker and traveled (on horseback and/or walked). The average person had hard physical work to do. Only aristocrats and many of the more well to do lived lavish, sensual, lazy lifestyles. I'm reminded of:
Philippians 3 (NIV)
17Join with others in following my example, brothers, and take note of those who live according to the pattern we gave you. 18For, as I have often told you before and now say again even with tears, many live as enemies of the cross of Christ. 19Their destiny is destruction, their god is their stomach, and their glory is in their shame. Their mind is on earthly things.
V. 19b "their god is their stomach" - (overeating and purging oneself was practiced by Roman governmental officials in their orgies and/or parties)
V. 19d "Their mind is on earthly things" - (not on the exercise/practice of godliness)
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Today
Unlike first century people, who may have had to walk miles just to get to work; we have cars, and many have "office" jobs or work that doesn't require much physical activity. This isn't true of everyone - (my physically-oriented job keeps me quite "buffed", though I'm currently laid off).
I have Christian friends who have "desk" jobs. Getting on their treadmill or walking/running outside and doing other "workout" stuff is a priority in their lives. They're just taking care of themselves, keeping in good condition, and so on. To my knowledge they don't take it overboard. I don't know what "overboard" might be for them. For myself, it would be if I spent more time and effort than needed to keep myself healthy - to the exclusion of being involved in other things that are of eternal (godly) value.
I suppose I should take a walk this evening...to stay in shape!
Thanks!
