The Christian can do what they want to do. Is that true? In answering that question we must bear in mind that to do sin is not good; in fact it is an abhorrence to God. So if the Christian wants to do sin they cannot do what they want to do.
But it is true, because a Christian wants to please God. He wants to go in the way of righteousness. The Christian does this because they have been rewired. Paul puts it to the Romans like this: But thanks be to God that, though you used to be slaves to sin, you have come to obey from your heart the pattern of teaching that has now claimed your allegiance. You have been set free from sin and have become slaves to righteousness (Rom. 6:17-18).
Moreover, we have been given the Holy Spirit so we now have the ability to please God. The Holy Spirit is God working in our lives to please God. So we have the fruit of the Spirit emerging “naturally” from our lives if we have the Spirit. But the fruit of the Spirit is love, joy, peace, forbearance, kindness, goodness, faithfulness, gentleness and self-control. Against such things there is no law (Gal. 5:22-23)
So the Christian now has been re-wired and re-empowered and that means they can do what they want. Moreover, in doing what they want they can know that it will not be bad.
We can also say that if a person does what they want and what they do is sin, then they are not truly Christian even if they actually call themselves “Christian”. However, if someone does sin and in sinning they are not doing what they want, they are showing themselves to be a true Christian and they need to repent.