All of these are words for people who oppose or disagree with a view, but they do have differences:
A dissenter is simply anyone who disagrees with some group on some opinion that that group otherwise shares. It does not have the strong negative connotations of the other two words - you can say someone dissents without expressing an opinion on whether they are right to do so or not. Even in a religious context, dissent is not always considered bad - there are many minor points on which some members of a faith disagree.
Renegade or rebel suggest that the person so described not only disagrees with the group, but has (perhaps violently) disassociated themselves from it as a result. While one can dissent from a group and still be a part of that group, a renegade has almost certainly either been kicked out of the group, or quit it because of their disagreement. This is particularly true in a religious context - a renegade member of a faith has a disagreement with the other members of that faith so profound that those others no longer consider them 'one of us'. Unlike 'heretic', renegades and rebels are not exclusively religious - one can be a renegade from a political party or a nation just as easily.
Heretic is very strongly associated with religious dissent. Originally, it would exclusively have been used in this way, and described someone whose opinions were so contrary to the established orthodoxy that the leadership of the faith declared that anyone who believed it was no longer a member of that faith. In modern usage, the word is used to deliberately evoke those religious associations. This can be either to express extreme distaste for the dissenter's view (they're so wrong that it's as bad as heresy) or to imply that the mainstream view from which they dissent is like a religion. Given these associations, 'heretic' is probably the strongest of the three descriptors.
Defiant is usually an adjective, and could be applied to any of the above nouns. Defiant means 'boldly resistant to or refusing to obey some authority'. A dissenter may or may not be defiant when presented with the official opposition to their view. If they are defiant enough, they may become a renegade, rebel, or heretic.