By definition, a person that believes the gospel (1 Corinthians 15:1-5).
The first two sentences in 1 Corinthians tell us what a Christian actually is.
1 Corinthians 15
Now I make known to you, brethren, the gospel which I preached to you, which also you received, in which also you stand,
by which also you are saved, if you hold fast the word which I preached to you, unless you believed in vain.
With the caveat that they must also receive the Holy Spirit also.
Romans 10:9-10
That if you confess with your mouth Jesus as Lord, and believe in your heart that God raised Him from the dead,
you will be saved; for with the heart a person believes, resulting in righteousness, and with the mouth he confesses,
resulting in salvation.
The gospel is so simple that mankind cannot twist it out of shape.