There is a difference between sinful thoughts and sinful actions. The most important difference is that your sinful thoughts (if never verbalized or acted out) only hurt you and your relationship with Christ. Sinful actions do that as well; but, they also hurt others, and even more importantly, they bring dishonor and shame to Christ's name.
If you love Christ then realizing this should keep you from outwardly sinning. If it does not, then, although you think you love Christ, you do not.
Our motivation behind not outwardly sinning should not be about proving to ourselves that we are in Christ. It should be about Honoring Christ! This is about Christ!--caring about his Honor and His Name as we go about living in this fallen world.
I read this Spurgeon quote this morning and thought "if every genuine born-again believer applied this simple truth to their heart and mind and actually brought it to mind in every situation that might tempt us to sin outwardly in action and behavior; we would never need to read an entire book devoted to teaching us "how to resist temptation".
Of course, dealing with the inward sins of the mind and heart is an entirely different matter. But, almost all outwardly manifested sins are "premeditated"; done with knowledge and can be prevented.
Is it really this simple? I believe that it is. It is for those who care more about Christ's name and honor then they do about acting out their own self-centered sinful desires. Read what Spurgeon says, and tell me if you disagree.
You are Christ's.
-- 1 Corinthians 3:23
"You are Christ's." You are His by donation, for the Father gave you to the Son; His by His purchase of blood, for He paid the price for your redemption; His by dedication, for you have consecrated yourself to Him; His by relation, for you are named by His name and made one of His brethren and joint-heirs. Labor practically to show the world that you are the servant, the friend, the bride of Jesus.
When tempted to sin, reply, "I cannot do this great wickedness, for I am Christ's."
When wealth is before you to be won by sin, say that you are Christ's, and touch it not.
Are you exposed to difficulties and dangers? Stand fast in the evil day, remembering that you are Christ's.
Are you placed where others are sitting down idly, doing nothing? Rise to the work with all your powers; and when the sweat stands upon your brow, and you are tempted to loiter, cry, "No, I cannot stop, for I am Christ's. If I were not purchased by blood, I might be like Issachar, crouching between two burdens; but I am Christ's and cannot loiter."
When the siren song of pleasure would tempt you from the path of right, reply, "Your music cannot charm me; I am Christ's."
When the cause of God invites you, give your goods and yourself away, for you are Christ's. Never contradict your profession. Be ever one of those whose manners are Christian, whose speech is like Jesus, whose conduct and conversation are so reminiscent of heaven that all who see you may know that you are the Savior's, recognizing in you His features of love and His countenance of holiness.
"I am a Roman!" was of old a reason for integrity; far more, then, let your argument for holiness be, "I am Christ's!"