Summary: In our modern day we as Christians can assume that we're fine with one foot in and one foot out. We've got the whole grace through faith in Christ equation going in our lives.
"Let us walk properly as in the daytime, not in orgies and drunkenness, not in sexual immorality and sensuality, not in quarreling and jealousy. But put on the Lord Jesus Christ, and make no provision for the flesh, to gratify its desires." -Romans 13:13-14 ESV
In our modern day we as Christians can assume that we're fine with one foot in and one foot out. We've got the whole grace through faith in Christ equation going in our lives. That's what it's really all about right? I mean, if we have some active sins in our lives, no big deal right? I mean, nobody can be perfect. Especially if it's just some of the 'small sins.' After all we've got grace, right?
Part of the equation of salvation is the recognition that we've set aside our old selves, which were corrupted by sin, and we've "put on" our new selves which are being built up in righteousness. Indeed, the equation of salvation is fairly simple, as Jesus put it, "repent and believe the good news" (Mark 1:15).
"Put off your old self, which belongs to your former manner of life and is corrupt through deceitful desires, 23 and to be renewed in the spirit of your minds, 24 and to put on the new self, created after the likeness of God in true righteousness and holiness." -Ephesians 4:22-24 (ESV)
And again, "Be holy as I am holy, for without holiness no one will see the Lord" 1st Peter 1:15-16.
Also it is written: "Make every effort to live in peace with everyone and to be holy; without holiness no one will see the Lord." -Hebrews 12:14
So, let's say I'm a Christian, and I live my life in service to God, preaching the gospel, and living a set apart life. But during my life I never repent (turn away from) the sin of stealing. I occasionally download music illegally from the internet. I do that in my life, then I die, and face God in the judgment at the end of the world. Do I go to heaven or hell?
Another example, we have a woman, shes a Christian, lives for God, and serves in lay ministry. But there's a sin in her life that she treasures, it's the sin of gossip. She loves to speak about others, and share details about their lives, and she can sometimes be harsh and cruel in her private judgments. So she dies, and she goes to face God on judgment day. Where does she go?
Another example, we have a man who is a Christian, but he holds out a fair amount of unforgiveness toward people who have hurt him. Other than that, he leads a pure life, but he feels justified in not forgiving others who have sinned against him. He dies suddenly, and goes before God. Where does he go?
Still another example, a man struggles with pornography and masturbation. He's a minister, and he preaches to a large congregation. He gives a good witness, but has a private addiction that he never addresses despite the Holy Spirit prodding him to do so. Where does he go when he dies and faces God?
One final example, a godly Christian woman has led dozens of people to Christ, she's done great deeds in His name, but she hangs on to a certain habit. She asks a friend at a local pharmacy to get pills for her, and give them to her, for a sickness she has, which she could pay for, but since her friend works there, she gets them for free. The Holy Spirit has convicted her many times, but she refuses to set those things aside. As she passes away, and faces God, where will she go? Heaven or hell?
The truth is that each of these people in these scenarios would not go to heaven, they would go to hell. Can a Christian end up going to hell? Yes they very much can. We each have a requirement in our personal lives of living in holiness and purity before God.
This is a high standard of living, but it's quite thoroughly livable in the power of the Holy Spirit. Thankfully for each of us, the Holy Spirit is active in our lives, convicting us, rebuking us, comforting us, and helping us live out our Christian walk. But we have the choice of resisting the Spirit's leading, and clinging to our old sins. And if we don't "repent and believe the gospel" but remain unrepentant (unwilling to change) we shouldn't expect to inherit eternal life. Instead we'll inherit condemnation.
Ephesians 4:30-31 "Do not grieve the Holy Spirit of God, by whom you were sealed for the day of redemption. Let all bitterness and wrath and anger and clamor and slander be put away from you, along with all malice."