When we accepted Christ as our Savior, we are immediately in a new relationship with God the Father.  God the Father is immediately our Father. He is Abba—Daddy—to us.  But, of course, if this is so, we should be experiencing in this life the Father’s fatherliness. When I accept Christ as my Savior, I also come into a new relationship with God the Son. He is at once my vine, my bridegroom.  Now this raises a question.  If I, as a branch and as a bride, am not bringing forth fruit one would expect from him, who is my vine and my bridegroom—what is wrong?   

Likewise, my brothers, you also have died to the law through the body of Christ, so that you may belong to another, to him who has been raised from the dead, in order that we may bear fruit for God.  Romans 7:4 ESV 

Notice the double ‘so that you’: first, that we ‘may belong to another’ to Christ; second, ‘in order that we may bear fruit for God.’  But with that must go the very sober warning:  

Do not present your members to sin as instruments for unrighteousness, but present yourselves to God as those who have been brought from death to life, and your members to God as instruments for righteousness. Romans 6:13ESV 

As a Christian, I can yield myself to one or the other, in order that I might be used by one or the other, as a weapon in the warfare that is being fought.   

For sin will have no dominion over you, since you are not under law but under grace.  What then? Are we to sin because we are not under law but under grace? By no means!  Do you not know that if you present yourselves to anyone as obedient slaves, you are slaves of the one whom you obey, either of sin, which leads to death, or of obedience, which leads to righteousness?  But thanks be to God, that you who were once slaves of sin have become obedient from the heart to the standard of teaching to which you were committed, and, having been set free from sin, have become slaves of righteousness.  I am speaking in human terms, because of your natural limitations. For just as you once presented your members as slaves to impurity and to lawlessness leading to more lawlessness, so now present your members as slaves to righteousness leading to sanctification. For when you were slaves of sin, you were free in regard to righteousness.  But what fruit were you getting at that time from the things of which you are now ashamed? For the end of those things is death.  Romans 6:14-21 

This passage points out our high calling, to put ourselves by choice in the arms of our rightful lover, our bridegroom, in order to bring forth his fruit in the external world.  But it also warns us that it is possible, even after we are Christians, to put ourselves into the arms of someone else and bring forth his fruit in this world.  If I as a Christian am not bringing forth the fruit that one would expect, the fruit of Christ, there is spiritual unfaithfulness on my part.  And when we see it this way, the word unfaithful takes on a very special and clear significance, for faith is the instrument by which we bear the fruit of our risen Christ. So the word faithless has a very pointed meaning. If I do not have faith toward Christ, I am unfaithful toward him, and this is faithlessness. 

Thoughts developed or used directly from the work of Schaeffer, Francis. True Spirituality . Tyndale House Publishers, Inc