So according to the Bible, Jesus said:
“Watch out for false prophets. They come to you in sheep’s clothing, but inwardly they are ferocious wolves. By their fruit you will recognize them. Do people pick grapes from thornbushes, or figs from thistles? Likewise, every good tree bears good fruit, but a bad tree bears bad fruit. A good tree cannot bear bad fruit, and a bad tree cannot bear good fruit. Every tree that does not bear good fruit is cut down and thrown into the fire. Thus, by their fruit you will recognize them. (Matt 7:15-20, NIV)
And, according to the Bible, Paul wrote this to the Galatians:
But the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness, gentleness, self-control; against such things there is no law. (Gal 5:22-3, NASB)
I believe these fruits of the Spirit cannot be faked.
We can tell the difference between unselfish kindness and acts done out of self-interest no matter who does them. And I believe that a kind atheist is kind through the same Spirit as someone who believes in the Spirit of God. Conversely I believe that people renowned for their Christian leadership who speak in arrogant, angry, uncaring ways, are no more controlled by the Spirit of God when they do that than anyone else who is being arrogant, angry or uncaring.
I believe it is worse to go against the very Spirit inside us than it is to appear to deny God or Jesus. That no long fancy creed or statement of doctrine will make up for speaking or acting from a wrong spirit.
And conversely I believe that anyone who is genuinely kind, no matter what they say they do or don’t believe, will not fail to be welcomed, if there is a day coming when that person is going to be face to face with God who literally is loving-kindness. They knew it when they saw it and one day they will be rewarded for that.
What of those who overtly knew of the Spirit of God and what fruit it produces and yet continued in arrogant, angry, uncaring ways? I don’t know how it will go for them, but I do believe God’s grace is sufficient even for them. I suspect they may find they are further down the rewards list than they expected. Because of the choices they made, when they had so much knowledge. But I believe they will still be on it because God keeps his promises.