You’re judging only by how things look on the surface. If someone is confident that they belong to Christ, they should consider again that we belong to Christ just as much as they do.
But this will happen in a particular order. Christ is the first of those who rise from the dead. When he comes back, those who belong to him will be raised.
If the Spirit of God truly does live in you, you’re not ruled by your desires—you’re ruled by the Spirit. Anyone who doesn’t have the Spirit of Christ doesn’t belong to Christ.