The professing Christian who, by misunderstanding Scripture and/or “the proper form of the doctrine” refuses to proclaim justification by faith alone.
OR
The professing Christian who twists Scripture or denigrates any passage he feels he needs to in order to “defend” his understanding of justification by faith alone.
I would say the one who is in more trouble is the one who pridefully makes an idol of his position and stubbornly refuses to listen to the direction of Scripture and the Spirit. And it could be either one of those, or both. People can willfully misunderstand and refuse to be corrected. And other people could do violence to Scripture out of a misguided desire to defend the character of God. Error is always dangerous, but not as dangerous as a sinful disposition.