If you sin, yes you are still a Christian. Paul, when writing part of the New Testament said 'i do what i do not want to do'. Meaning, even as a Christian, and a writer of most of the NT, he was still sinning, still messing up. While our spirits are reborn, our physical bodies are still stuck in this sinful world, with sinful habits and inclinations. We battle against our flesh. And this is the essence of our faith. Resisting the wrong our flesh desires by empowering our reborn spirit to fight our sinful desires. Sometimes we lose. We just seek forgiveness and move on. God forgives.
But, if you choose to live in a sinful lifestyle, accepting your sin or sins, and not desiring to change, then you might find yourself questioning your salvation. The desire to try to fight against sinning and doing Gods will is part of the evidence of our salvation.