On Who Is or Is Not a Christian

I sometimes log into my church's website under one of the pastors' name. I do this because I'm the webmaster and I sometimes act as a cyber-scribe, writing little blurbs for the Sunday sermons, which we offer to stream on the site. I had adapted this feature from a pre-existing feature called "From the Pastor's Desk" when I revamped the site and it just seemed like a good idea at the time. But now, when I log on as one of them and make an entry, nobody knows that it isn't them. I do my best to write something I think they would have written had they the time, and I always notify them right away and ask for any changes they might like. They have always let my writing stand. But meanwhile nobody else has any idea that it was I who acted in the pastor's name. I find this a daunting and instructive experience.
When someone in the world invokes the name of Jesus, the rest of the world generally is perfectly happy to blame Jesus himself for whatever that someone does or says. In fact the world generally assumes that if you voluntarily say you're a Christian, then you must be one. It's like saying you're a virgin: virgin's don't lie.
At this point, let me offer a fairly simple, though impossible-to-check, definition of a Christian. It is someone who believes that Jesus is the only begotten son of God and that God has raised him from the dead - and coming to this belief, this person surrenders his own life, subordinates his will to the best of his ability, to the living Christ (that's what they call "repentance" or turning around). That's it in a nutshell.
This belief and surrender transforms us immediately, spiritually (the idea of being born again and being baptized into Christ), but we bring to this moment a lifetime of decisions and actions that do not reflect this belief. Fortunately, God is willing to provide the therapy we need to remediate our minds and bodies, increase our trust in Him, deepen our relationship with Him. As we pursue this loving God, we will make choices that confound the people around us. We will still make mistakes and do things that do not reflect well on the name of Jesus, and that makes it all the more tricky for anyone else to be sure of our standing.
Because this all takes place in the heart, it is 1) pretty easy to know for sure whether you yourself have taken that step; and 2) pretty much impossible to know for sure whether anybody else has.
Some people seem to resent the very idea that someone is or is not a Christian. Surely this all-or-nothing thinking is simplistic, even dangerous, us-against-them? If people are nice and go to church, maybe give money to charities, like doesn't that count for something?
Jesus says that not everyone who calls him Lord will enter into the kingdom of heaven: what counts is doing the will of the Father. Some people may have spectacularly successful - by all appearances - "Christian Ministries", even prophesying and working miracles; and He will say to them "I never knew you, depart from me you evildoers."
Faith in Jesus is not the only motivation by which people do charitable works, and it is certainly better for the world for people to engage in charitable works than in selfish ones; but God, who sees the heart, cares more about having a personal relationship with each of us than anything else I can think of. Charitable works are all very nice and in fact our relationship with Him, if it has any reality, will motivate us to such works. But citizenship in the kingdom of heaven is only available by birth.