How do we know who has the Spirit of Christ?

While anyone can claim to be in Christ (i.e., a born-again Christian), only those who have the Spirit of God are truly of Christ. For example, in Acts 8 we learn of a man who desired to be associated with Christians and Christianity, but he didn’t possess the Spirit by faith, and therefore he was deemed to be an unbeliever:

Acts 8:18 Now when Simon saw that the Spirit was bestowed through the laying on of the apostles’ hands, he offered them money, 
Acts 8:19 saying, “Give this authority to me as well, so that everyone on whom I lay my hands may receive the Holy Spirit.” 
Acts 8:20 But Peter said to him, “May your silver perish with you, because you thought you could obtain the gift of God with money! 
Acts 8:21 “You have no part or portion in this matter, for your heart is not right before God. 
Acts 8:22 “Therefore repent of this wickedness of yours, and pray the Lord that, if possible, the intention of your heart may be forgiven you. 
Acts 8:23 “For I see that you are in the gall of bitterness and in the bondage of iniquity.” 

It's not surprising that men claim to have the Spirit even when they do not. The unbelieving world cannot understand what they do not possess, so from their limited perspective, they can only assume that Christianity is an association or relationship with other people, when in reality it is a spiritual relationship with the living God through His Son, Jesus Christ.

So how do we know if a person is truly Christian? Our ability to discern true faith in another individual will always be limited and prone to error since true faith is a spiritual (i.e., invisible) relationship. Nevertheless, scripture teaches that true faith produces fruit by the Spirit in the life of a believer:

Matt. 7:15  “Beware of the false prophets, who come to you in sheep’s clothing, but inwardly are ravenous wolves.
Matt. 7:16 “You will know them by their fruits. Grapes are not gathered from thorn bushes nor figs from thistles, are they?
Matt. 7:17 “So every good tree bears good fruit, but the bad tree bears bad fruit.
Matt. 7:18 “A good tree cannot produce bad fruit, nor can a bad tree produce good fruit.
Matt. 7:19 “Every tree that does not bear good fruit is cut down and thrown into the fire.
Matt. 7:20 “So then, you will know them by their fruits.
Matt. 7:21  “Not everyone who says to Me, ‘Lord, Lord,’ will enter the kingdom of heaven, but he who does the will of My Father who is in heaven will enter.

Jesus warns that we may know people by their (spiritual) fruit. Likewise, Paul says the fruit of the Spirit in the life of a believer will stand in stark contrast to the easily recognizable sin in the life of an unbeliever:

Gal. 5:19 Now the deeds of the flesh are evident, which are: immorality, impurity, sensuality, 
Gal. 5:20 idolatry, sorcery, enmities, strife, jealousy, outbursts of anger, disputes, dissensions, factions, 
Gal. 5:21 envying, drunkenness, carousing, and things like these, of which I forewarn you, just as I have forewarned you, that those who practice such things will not inherit the kingdom of God. 
Gal. 5:22 But the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness, 
Gal. 5:23 gentleness, self-control; against such things there is no law. 
Gal. 5:24 Now those who belong to Christ Jesus have crucified the flesh with its passions and desires. 
Gal. 5:25 If we live by the Spirit, let us also walk by the Spirit. 

As we see spiritual changes taking place in a believer's life, we can gain confidence in their testimony of faith in Jesus Christ. On the other hand, how much fruit is enough to satisfy our inspection? This question highlights the dangers of becoming "fruit inspectors" within the body of Christ. Our inherent inability to perfectly discern true faith among very immature Christians should cause us to refrain from such judgments in most cases. Only in situations where a person is seeking a leadership or teaching positions within the body should we take up the challenge of validating their testimony of faith. 

For a more teaching on this topic, see related questions.

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