This is a pretty easy question since it's defined in scripture.
A Christian is one who hears the gospel of Jesus Christ, believes it to be true, believes that Christ was God incarnate and that, only through his ultimate sacrifice, is the penalty of sin atoned for, including one's own personal sin. A Christian is one who knows this and repents of their sin (meaning they acknowledge it as hurtful towards their relationship with God, a intrinsic part of their separated nature, and that only God can remove it from their lives, and they CHOOSE to live a life in a manner that glorifies God and rejects sin), chooses to trust God with the task of cleansing it from their lives, and to transform their heart to love Him and others more and more every day. Being a Christian doesn't mean you're perfect. In fact, the whole doctrine relies on the imperfection of human beings and how NO ONE has made themselves righteous in the eyes of God, because as loving as God is, he's also a judge, and good judges can't ignore evil. Being a Christian means that you honestly want to love and be more like Christ was every single day, and that you can only achieve that through submission to God, receiving the Holy Spirit, and meditating on His Word (The Bible). Being a Christian is NOT "behavior modification", it is a transformation of the heart and mind and its will, behavior naturally follows. Being a Christian is all about being madly in love with the Creator who made you (in Biblical standards, marriage between man and woman is a partial image of the relationship between Christ and the Church), knew your wretchedness, and died to save you, because you were that important to Him. Thus, it's not just an acknowledgment of the gospel, it is an acceptance of the Holy Spirit and a decision to walk a new life in God's direction, not sin's direction. Christians will sin, but our sin will not land us in hell, as we've already been purified by our decision to let Christ be our bathwater. Nothing can separate a Christian from God's reach. But we should not seek to sin, as if we still seek to live in a manner that feeds our fleshly, sinful desires, then one must stop and evaluate if they truly love God (because sin hurts God) and truly chose to accept His sacrifice and live for something better. You can't fool God, but you can fool yourself and others. Being a Christian is a matter of the heart and the decision of which master you will serve, yourself, or God. That's why Christ says that one will know his disciples based on their ways, because Christians are meant to be different from the world's standards, even to the point of death. But Christians are NOT saved based by their deeds, they are saved through faith alone in Christ and his work. Good deeds are simply proof of faith perfected in the Christian. Christians also live with the hope that they will unite with God in Heaven, and that a day is coming when evil will be wiped from the earth and all will be judged according to whether they received the gospel or not (which is why the day has yet to come, as God's promised not to return until the gospel has been preached to every corner of the earth, because He wants as many as possible to come to salvation).
There's a bazillion pieces of scripture that refer to all this and I could post them, but you can find it yourself too
Blue Letter Bible - Home Page
Also, I don't think Judas was saved. Not only did he betray Christ, he didn't repent of it. He seemingly had no remorse. Scripture revealed his bad character throughout the whole gospel. He and Jesus both knew what was going to happen, and he still chose to go along with it. That happens to all of us sometimes, when we sin, but he wasn't even sorry. Then he took his own life.
(Luke 22:3) It says that Satan came inside Judas. Judas gave the devil a cozy home in his heart because of his evil desires.
And yeah you guys are right. The devil attacks Christians way more than those who are already on his side.
