When we think about what true Christianity looks like, we cannot merely consider the way in which people who profess to be Christians behave. There are criticisms that are often made of things done by the Church which are absolutely valid. Perhaps one of the most common criticisms is of the various Inquisitions, for example. But in many cases, things done in the name of Christianity were not done by true Christians at all, but wicked people who used religion as a means to an end. And yet even some un-Christian things have been done by genuine Christians who have been misguided, or allowed sin and pride to get the better of them. So when we ask “What does true Christianity look like?”, we're not trying to create some perfect litmus test with which we can determine whether a person is truly saved or not. Rather, we're trying to give a general idea of what the Christian life ought to be like, according to the way God desires us to behave. Jesus said, “You therefore must be perfect, as your heavenly Father is perfect.” (Matthew 5:48). Well, we all fall short of that! The very reason Jesus said this was for us to realize that we cannot be perfect, and that we need a Saviour.
Having said that, Jesus did say of false Christians that we shall “know them by their fruits.” That is to say, a person's behaviour will naturally express what their heart is made of. Just as every lie is eventually uncovered, a person cannot fake love and kindness indefinitely without their true nature eventually showing through. Non-Christians certainly are kind and loving, and genuinely so, because we are all made in the image of God. However, the source of a Christian's love is a conscious awareness of this fact; that God has made us all, even our enemies, and that He loves and values us all. The love of a Christian ought to be more far-reaching than the "common love" of men, as God's is. And it should be consistently genuine as it is a part of our new nature, given to us by Christ.
The epistle of 1st John was written to a Church in order to speak about some false Christians who had been members of the congregation, and it talks a lot about the difference between a true Christian and a false one. But it begins in the latter part of the first chapter by saying this:
If we say we have no sin, we deceive ourselves, and the truth is not in us. If we confess our sins, he is faithful and just to forgive us our sins and to cleanse us from all unrighteousness. If we say we have not sinned, we make him a liar, and his word is not in us. (1 John 1:8-10)
This is the first distinction between believers and unbelievers. Nobody is without sin. But the way we deal with our sin and our guilt is very different when we're a Christian. The unbeliever will tend to justify themselves when they do wrong, or they may even blame and hate themselves. But a Christian will confess their sin to God in repentance, owning their sin and yet finding forgiveness to move forward.
In chapter 2, John begins to talk about another key distinction between true and false Christianity:
And by this we know that we have come to know him, if we keep his commandments. (1 John 2:3)
Now, John has just said that we are all sinners, and yet a true Christian keeps Christ's commandments. He means that it is our constant desire to obey them, though we fail to do so perfectly. That desire affects our behaviour so that we do actually go about our lives in obedience to Christ's commandments. All people do genuinely do good because of the moral sense God has given to all mankind, but an unbeliever doesn't generally understand this, and so they fail to give God the glory in the works that they do. But a Christian is consciously desiring to do the will of God in everything they do.
The major Biblical distinctive for Christians is love. Jesus said “By this all people will know that you are my disciples, if you have love for one another.” (John 13:35). John makes this a strong point in his epistle, too:
For this is the message that you have heard from the beginning, that we should love one another. … But if anyone has the world's goods and sees his brother in need, yet closes his heart against him, how does God's love abide in him?” (1 John 3:11,17)
The love that a Christian has for others comes from God. We love others because God is expressing His love for them through us. This is why it is impossible for a Christian not to love others. “We ought to love others in the same way that a fish ought to swim,” as I heard a preacher say once. John says in chapter 2 “Whoever says he is in the light and hates his brother is still in darkness.” (1 John 2:9). Now because we are all made in the image of God, we all do experience love for others. But it is easy to love those who love us, or if it won't cost us anything. To even love one's enemies, or to sacrifice much for the sake of another; that is the love of God in us. The Christian life is characterized by this kind of love.
The final point which I'd like to highlight from 1 John is this... “Whoever confesses that Jesus is the Son of God, God abides in him, and he in God.” (1 John 4:15). Believing that Jesus Christ is the Son of God, and the Saviour of mankind, is absolutely essential to being a genuine Christian. In fact, all that we've said so far illustrate a life where Christ is central; where He is the motivation for all that we do, and the very source of our love and our desires. Unless you have that, all of your love for others and your moral lifestyle are not sufficient to save you. John reminds us of this...
Whoever has the Son has life; whoever does not have the Son of God does not have life. (1 John 5:12)