Is the idea 'I can be whatever I want to be if I just think it' contrary to Scripture?
Yes, that motivational claim is contrary to Scripture. A believer never takes the approach that he can be whatever he wants by merely thinking it. Instead, Scripture says that in himself he is nothing, and only in Christ can he become anything, because the old Adamic nature has nothing good.
Les teaches plainly that this idea is contrary to Scripture. He recalls warning a believing friend who got wrapped up in motivational tapes, telling him he was on thin ice, because a believer never takes the approach, "I can be whatever I want to be if I just think it." A believer has to say, "in myself I am nothing," but "in Christ, then yes, I can become anything." Les then points to Paul's words that from the old Adamic nature's point of view there is nothing good: "For to will is present with me, but how to perform that which is good I find not." He notes that only one reference to the Holy Spirit appears in the first seven chapters, while chapter eight will mention Him many times and will answer the question of how to do what the new nature wants when the old nature keeps tipping the balances. Paul repeats it in verse 19: the good he would, he does not; the evil he would not, that he does. Les adds that some people even claim they no longer sin, having reached what they think is the pinnacle of Christianity.