Many true ways to answer this question. Here is one. A Christian is a man or woman whose deepest heart desire is God himself.
Scratch below the surface desires and needs of life, such as food and companionship and so on and get to the bottom level of desire. What do you find? A Christian is a man or woman who desires God. "My soul finds rest in God alone" is how the psalmist put it.
This, we must insist, is so different from the world that it requires a supernatural heart operation. Most people are quite happy with the desires of food and drink and companionship and pleasure.They may well realise that these things fail at the deepest level, but they have no desire for God. But a Christian has been 'born again' and transformed at the core level of their being so that now they long for God more than any other thing or one.
And they discover that God is more than able to satisfy the deepest desires of their hearts and so daily to renew their strength.