You might think that any Theist who believes that God has spoken through the Bible and that Jesus is the divine Son of God who rose from the dead must be a Christian. Strictly speaking, however, that isn’t the case, because there’s at least one more idea that is central to Christianity and distinguishes it from other religious worldviews—including the one we’ve ended up with here: Pelagianism.
Pelagianism gets its name from Pelagius, a monk who lived in Rome in the fifth century. Pelagius taught that humans enter this world completely untainted by sin and with perfect freedom to choose between good and evil. We aren’t born sinful, but we can become sinful by failing to follow God’s moral laws and Christ’s moral example. Pelagius held that heaven is basically the reward for a good life. If you follow the example of Jesus and live a good moral life, you’ll receive eternal life. God’s help (“divine grace”) is available for those who need it, but the basic principle is that the way to get to heaven is by doing your level best to love God and love others. In other words, divine grace isn’t strictly needed to get to heaven—and the less you fall back on it the better!
Pelagius claimed to be following the teachings of Jesus, but his views caused an uproar in the early Christian church because, in effect, he was denying the clear teaching of the New Testament that salvation is entirely a free gift of God. According to the Bible, we’re saved “by grace alone” and not because we live good lives.40 As the apostle Paul put it, sin earns us death, but eternal life is the gift of God in Jesus Christ.
Unpopular though the idea may be today, the Bible teaches the polar opposite of Pelagianism: we are born in a sinful and spiritually dead state, unable to live good moral lives apart from divine grace. Indeed, Jesus himself preached the shocking idea that heaven is for immoral people who admit that they’re bad and cry out for God’s mercy and forgiveness rather than for moral people who think they’re good enough to deserve it.
It’s not so surprising, then, that Pelagianism was condemned as heresy by the entire Christian church at a council in AD 431. Remarkably, however, there are many people today who consider themselves Christians but are actually closer to Pelagius when it comes to their beliefs about who gets to heaven and on what basis. Pelagianism says that we’re all born good, and if we stay good enough we’ll receive eternal life as our reward. In stark contrast, Christianity says that we’re all born bad, but bad people can still obtain eternal life—not because we deserve it in the least, but because out of his great love and mercy God offers it as a gift to those who put their trust in Jesus Christ.
(from James Anderson's "What is Your Worldview?")