What is a Christian?

As far as I can tell, after thirty years of experiencing God and life as a "Christian" and after thinking about this experience from several perspectives, a Christian is...
a person who has a personal heart-to-heart relationship with the living God, characterized by warm and active acceptance on God's part; our honesty and dependence on the activities of Jesus Christ.
Let's look at this a little more closely.
"A personal heart-to-heart relationship":
The point of this is to exclude 'religious' relationships, in which an 'object' is revered 'from afar' but not approached in a personal way. God is indeed an 'awesome entity' but he is nonetheless a Person. A personal relationship is a reciprocal relationship, not a one-way deal. There is real interaction, real disclosure, real closeness that develops. The 'heart-to-heart' aspect intends to convey the honesty and openness of this relationship. There are no 'games' that can be played with an all-knowing God(!), no secrets withheld, no area of life concealed. (The interesting thing about this is that, even though God knows all about an area of our life, we might NEVER open it up to Him in discussion, in our efforts to 'hide' from His feedback!)

I cannot emphasize strongly enough the personal character of this relationship. I see so many aberrations and stunted-growth versions of it. It is not a formal relationship, a primarily legal one, or even simply a 'creature-Creator' relationship. (I find the human tendency to relegate God into a religious icon or image or object to depersonalize the relationship and short-change the possibilities of such a relationship--much as we do in other significant personal relationships in our lives.)

"The living God":
 The subject of God is quite a vast one, but the main point here is that He is LIVING. There are feelings, and thoughts, and decisions, and actions, and initiatives, and responses, and values, and commitments... all the aspects of personal existence. He is not a force or an attitude or a "perspective on the universe". We walk around our lives 'face to face' with this One-- even if we ignore Him.
"Characterized by warm and active acceptance on God's part":
 From God's side, He accepts us. But this is not merely a 'political' acceptance--it has a warmth and joy to it. He 'smiles' upon us. He delights in us(!). This is more than simply the very important 'peace with God'; it is an active relationship. He gets involved in our lives for good--for our growth, our development, our character, our fulfillment, our stability, our significance in the lives and futures of others. He is always 'glad to see us'.
"Characterized by our honesty and dependence on":
 From our side, the relationship is one of honesty about who He is and who we are. We are not 'gods', and as such need our Maker for the realization of the purposes for which we appear in this universe. We are a people dependent on the universe He has produced, and we are people whose goodness has been severely compromised by our regular moral failures and pervasive spiritual apathy.

The main thing in the universe that God the Father loves...is God the Son. When we are honest with the Father about who his Son is, and what he did in history for us, God welcomes us into this warm relationship...We simply have to be honest with Him about his dearly-loved Son.

The second part of this is dependence. We depend on Him for the 'repair' of our relationship WITH Him. He is the active one, coming in history to earth and taking upon Himself the consequences of our moral failure. We simply are honest about those actions/events to the extent that we rely upon those actions/events as an adequate basis for God's warm acceptance of us. In other words, we agree with God that his Son's life and work are sufficient grounds to accept us into this special relationship. It's that simple.

"The activities of Jesus Christ":
 The basic 'core' truth of Who he was/is and what he did are simple. He was God the Son, who took on human flesh, lived among us, suffered at the hands of His Father (on the cross) as our substitute, came back to life after his execution, and transported himself 'outside' space-time to 'heaven'. He will return to earth visibly in the future, but for now, He is involved invisibly in the macro-forces of history, and the micro-events of our lives. His death satisfied God the Father's just moral demands upon us, and 'freed' God to lavish his warm acceptance upon us.
This is the beautiful truth of what a Christian is...a beloved child of the living and loving God...and it starts with a simple conversation with God...telling Him that you accept "His version" of who his Son was, and what He did for you...

For just a tad more detail on the "WHAT and HOW", without going into too much intellectual musings, you might look here

From: The Christian ThinkTank...[https://www.Christianthinktank.com] (Reference Abbreviations)