Most of us think about our spiritual lives (when we think of them at all) as cerebral, passive things: things that just are, likely supported by some “quiet time”, perhaps some Bible reading, and prayer.
However, our spiritual strength is only truly developed when, like a muscle in our body, it is stressed, and stressed repeatedly.
Most of us have no lack of stresses to resist against: health issues, relationship issues, financial issues. All of these stress our spirit and do their best to defeat our confidence and our peace. But, when we assert our presence in Christ against them, push back against them, we grow stronger.
We need to resist these spiritual stresses if we are to become fit for challenging and defeating the apathy, apostasy, and in some cases outright hostility, that surrounds us all.
NT Wright on Spiritual Training
In his “The New Testament for Everyone”, NT Wright, expounding on 1 Tim 4:10 (“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”), has this to say about striving in spiritual work:
“Verse 8 is one of the clearest references to physical exercise in the New Testament. As in the previous passage, there are echoes of 1 Corinthians here, in this case of 9:24-27; there, Paul mentions athletics, wrestling, running races, and boxing, all familiar sports in his world. Whether Timothy was actively involved in that kind of thing we can’t tell, since verse 8 may just be a powerful illustration rather than a comment on what he’s actually doing, but the point is obvious: for genuine godliness, true piety, you need to go into training just as much as an athlete does. And this sort of training is even more worthwhile. The first will make you physically fit, able (at least in principle) to work harder and enjoy life more. The second will make you… well, not just spiritually fit (as though the point of it all was to be able to engage in a more energetic spiritual life), but the kind of person who reflects God’s image, one who has taken him- or herself in hand, has seen the need to develop properly as a fully human being, and taken appropriate action. This, as we shall see presently, is what Paul means by life.
This is emphatically not what people today expect or want to hear. We expect and want to be told that ‘spirituality’ is simply the sense I have of being in God’s presence, being surrounded with his love, sensing a transcendent dimension in the affairs of everyday life. It comes as a shock to be told that it’s something you have to work at – and something, moreover, which will take the same kind of hard work as going into training for athletics, or even in order to move house.
Paul doesn’t say what kind of exercises he has in mind, though many wise guides have developed such things. But he does refer, in verse 10, to his own hard work and struggles, and the word ‘struggle’ is the regular one wrestlers would use. This is how Paul understands the work of prayer, pastoral care and evangelism: not as a smooth, easy set of tasks, the kind of thing that just flows naturally, but as something through which one is changed the way that a block of marble is changed, as the sculptor chips away at it to get to the beautiful statue she has in mind, and as something through which the world around is changed, in the way that a hard-working labourer can transform a plot of thistles and nettles into a lovely garden.
The garden he’s working on – or, if you prefer, the sculpture he’s chiseling out – has a grand name: life (verse 8). The power of death, decay, and deconstruction is so strong that if life, the new life that God longs to give to his whole creation, is to win, it must involve and engage all the energies of God’s people in working for it. This God is the living God (as opposed to the lifeless, powerless gods of popular mythology). Those who struggle and wrestle in their spiritual exercises under his direction are doing so in order to attain the ultimate life of the new world itself, and also the anticipation of that life which comes forward to meet us in the present.”
Some Useful Verses
The following are some verses you can use to meditate on, and be encouraged by, to build spiritual fitness in your life:
1 Timothy 4:7-8
 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.
 For the moment all discipline seems painful rather than pleasant, but later it yields the peaceful fruit of righteousness to those who have been trained by it.
 Therefore lift your drooping hands and strengthen your weak knees,  and make straight paths for your feet, so that what is lame may not be put out of joint but rather be healed.
1 Corinthians 9:25-27
 Every athlete exercises self-control in all things. They do it to receive a perishable wreath, but we an imperishable.  So I do not run aimlessly; I do not box as one beating the air.  But I discipline my body and keep it under control, lest after preaching to others I myself should be disqualified.
2 Timothy 3:16-17
 All Scripture is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness,  that the man of God may be complete, equipped for every good work.
 Whoever gets sense loves his own soul; he who keeps understanding will discover good.
 But solid food is for the mature, for those who have their powers of discernment trained by constant practice to distinguish good from evil.
 And let us consider how to stir up one another to love and good works,  not neglecting to meet together, as is the habit of some, but encouraging one another, and all the more as you see the Day drawing near.
 He gives power to the faint,
and to him who has no might he increases strength.
 Even youths shall faint and be weary,
and young men shall fall exhausted;
 but they who wait for the LORD shall renew their strength;
they shall mount up with wings like eagles;
they shall run and not be weary;
they shall walk and not faint.
1 Corinthians 10:31
 So, whether you eat or drink, or whatever you do, do all to the glory of God.
 I can do all things through him who strengthens me.
 Now to him who is able to do far more abundantly than all that we ask or think, according to the power at work within us,  to him be glory in the church and in Christ Jesus throughout all generations, forever and ever. Amen.