One of the seeming contradictions of the Christian faith is that you cannot work to earn salvation – the only way you can get salvation is by the free gift of Jesus – but if you do not work you will end up spiritually flabby. Spiritual exercise is something we do out of gratitude for what Jesus has done for us, and so that we can accomplish the parts that God has prepared for us in his plan for the world. Paul says this in the fourth chapter of his first letter to Timothy:
Stay away from godless stories and old wives’ tales. Train yourself in godliness. Physical exercise has some value, but godliness is valuable in all respects, offering promise both in the present life and in that which is to come. This saying is true and deserves to be accepted. We work and exert ourselves because we have put our hope in the living God, who is the Saviour of all people, especially those who believe. Teach these things and tell people to obey them.
According to Paul, the reason we work and exert ourselves in gospel work is because we have put our hope in the living God. Jesus has redefined the purpose of our lives from self-fulfilment to fulfilment of God’s desires. It is our privilege to have exciting roles in the work of extending the Kingdom of God on earth, and spiritual exercise helps keep us effective.