According to the letter of 1^st John, love is perfected by loving those who have the spirit of God. 

1 John 4:12-13 (KJV)
¹² No man hath seen God at any time. If we love one another, God dwelleth in us, and his love is perfected in us.
¹³ Hereby know we that we dwell in him, and he in us, because he hath given us of his Spirit.

This is a key to how we know who is truly from God and who is a false believer.  Many can recite and sermonize scripture to the point that discerning the imitations from true believers can become very difficult, especially for the new Christian.  I have seen fast-talking evangelists use scripture to charm potential believers into their flock like a modern-day Pied Piper.  As a matter of fact, let me remind you that it was the Devil who quoted scripture to Jesus during his temptation as recorded in Matthew 4:1.  The early church also faced the same problems with false prophets who saw an opportunity to obtain power through the acquisition of the Holy Spirit (Acts 18:18-19).  But sooner or later the false prophet reveals himself and his true motives, whether it be power, wealth, or fame. 

God’s love is not some Hallmark saying that simply sounds poetic to those who hear it. If it were, there would be little difference between truth and deception.  God’s love, however, is a moral love and a desire for a moral life in Christ.  One cannot love Christ without loving who Christ was, what he did, and what he taught.  Jesus was not and is not an abstract concept of God that exists only in the minds of men.  He is a personalized revelation of God.  So, when John tells us to love one another he is telling us to love this moral image that we have of God as we see him revealed in Christ and in our brothers and sisters.  Likewise, it is this moral love that dwells within each of us that transforms us into the image of Christ and helps us to recognize this image in others.  This is the power of the Holy Spirit. 

Now I once heard someone say that once they learn how to imitate sincerity that they’ve got it made.  Unfortunately, I do believe that it is possible for someone to imitate sincerity because I have actually seen people do it. So, looking and sounding sincere is not what makes a person a true believer; it is their love for God, for Christ, and for the believer as shown in their behavior and attitude that is critical as evidence of true belief. 

Jesus said:

Matthew 6:19-23 (KJV)
¹⁹ Lay not up for yourselves treasures upon earth, where moth and rust doth corrupt, and where thieves break through and steal:
²⁰ But lay up for yourselves treasures in heaven, where neither moth nor rust doth corrupt, and where thieves do not break through nor steal:
²¹ For where your treasure is, there will your heart be also.
²² The light of the body is the eye: if therefore thine eye be single, thy whole body shall be full of light.
²³ But if thine eye be evil, thy whole body shall be full of darkness. If therefore the light that is in thee be darkness, how great is that darkness!

No matter how sincere or heart-warming a person can quote or preach the word of God they cannot hide where they keep their real treasure.  So, watch carefully where others keep their treasure and watch out for yourself as well. 

If you love God and are transformed by that love, then his love is perfected in you. And where love is perfected fear is cast out.  For there is nothing that a believer ought to fear since all things are working out for good. 

Romans 8:28 (NKJV)
²⁸  And we know that all things work together for good to those who love God, to those who are the called according to His purpose.

All sorrow and pain are temporary, but love is eternal.  So, there is no need to fear for fear comes from uncertainty and uncertainty is a product small faith. 

1 John 4:18 (KJV)
¹⁸ There is no fear in love; but perfect love casteth out fear: because fear hath torment. He that feareth is not made perfect in love.

Paul taught us that perfect love will produce both perfect faith and perfect hope.  So, meditate upon perfect love and everything else will follow.

1 Corinthians 13:13 (ASV)
¹³ But now abideth faith, hope, love, these three; and the greatest of these is love.