—2 Timothy 2:4-7 (NASB)
Soldiers are like Christians in that each has a "calling", so to speak. After basic training, each soldier is assigned a specific job for which he is not only thoroughly trained but then assigned to a specific place to perform that function on behalf of the greater whole. Whether they're an infantryman on the front line or a supply clerk or a company First Sergeant, every part of the whole military machine is dependent on each individual part fulfilling its assigned duties as proscribed. It's a full-time endeavor. Christians shouldn't look at their "gift set" or "ministry" as a part-time or weekend only thing, but as their primary role that they may please God who called and assigned them.
Athletes have great restrictions in terms of diet, training, and the rules of competition. For their particular sport there may be weight classifications, gender separation, legal and illegal methods, and a myriad of other rules and regulations. They're not considered fit nor ready to even show up and compete—much less actually win—if they have not subjected themselves voluntarily to these boundaries. Christians are called to live righteously; that is, according to God's rules and regulations. They are not allowed to "compete" in any and every condition according to however they want. They must adhere strictly to His rule book: The Bible.
Farmers labor to produce a harvest that not only benefits the many that are fed, but for himself as well. He is the first to experience the joy and satisfaction of what he planted, nurtured, and harvested. It's a built-in motivation to work as hard as possible because he not only provides for the needs of many, but for his own self as well. Christians are the first to reap the spiritual rewards of a community and world in which they've planted and harvested the Gospel, not just in the life to come but in the affect on their surroundings in the here and now.
Biblically, these illustrations are often known by the labels sanctification, obedience, and perseverance. The trait common to all examples is what the Bible calls "faith", which can also be translated "faithfulness"; that is, to consistently live committed to HIS ways each and every day to produce the fruit of the kingdom of God. Let's each enlist, submit, and follow through. Full time.†††