What is a Christian?

Good question! People have been asking this question for about 2000 years now. The short answer is : A Christian is a person who follows Christ. Perhaps you were hoping for something a little deeper?
You actually have to start this answer way back since before God created this universe for us.

We can’t go back to a time before God existed, because He has always been and will always be, and our puny human brains will never be able to understand how that could be.
Before God created people, He created angels. (Despite a common misperception that humans become angels after we die, angels and humans are not the same creatures at all.)

One thing that we have in common is free will – we can choose to serve God or choose to go our own way. Angels, however, have already had their judgment day. A beautiful, majestic angel named Lucifer was jealous of God, and wanted to be like God. As a result, Lucifer was cast out of Heaven, and about one third of the angelic population chose to follow him. They are now known as Satan and his demons. I’m not sure where they live right now, but apparently they are free to visit Heaven and wander around in our universe.
When God created our universe and made two people to start the human population, He offered free will to them, allowing them to choose between eating fruit from the Tree of Life or the Tree of the Knowledge of Good and Evil.

As always, God offered the choice between life and death, and encouraged Adam and Eve to choose life. Satan, however, has always known that at the end of human history he is going to be cast into some place called The Abyss, and God will then lock him and his followers there forever. In retaliation, Satan tempted Adam and Eve to choose eternal death with him. His goal has always been to take as many human souls down with him as he can. By succeeding in his goal to get Adam and Eve to disobey God, Satan won the authority to be the “Prince of this world”.

I don’t make any claim to understand spiritual politics ( I don’t even understand human politics), but because of this authority, all human souls belong to Satan by default unless they make a deliberate decision to accept the ransom provided by Jesus Christ when he gave his own perfect body as a sacrifice to redeem our souls. (The exceptions to humans belonging to Satan by default are babies and children who have not yet reached the age of accountability, which most theologians believe to be age 12.)

This is turning out to be a very long answer to a short question, but discovering the answer to the question “What is a Christian” is of eternal significance. Unfortunately, our souls belong to Satan unless we choose to accept Christ’s redemption. There is no neutral position. The good news is that becoming a Christian is very easy. We confess that we are sinners, unable to earn our own salvation. We accept the belief that Christ died in our place in order to ransom/redeem us from Satan. Then we ask for forgiveness for our sins, and for him to “come into our hearts”.

Another common phrase for this choice is to be “born again”. Whatever term you use, it means that somehow our souls are reconnected with our Creator, and His spirit “indwells” with our spirits. My opinion (from my fallible human brain) is that what we call our “conscience” is actually the connection between our souls and God. We can choose to ignore our conscience, and eventually that connection is severed and seared. If we make the choice to let God indwell us, His spirit communicates with our spirits through a “still, small voice” that is usually inaudible.
“When you look for God, He will always find you.”