I think the key word here is "inclination". It's not that the people never had a good thought, but what were they inclined toward? What was their gut response, their initial attitude? The Bible tells us that they were inclined toward evil.
How did they get this way? What happened to make their first thought toward anything be the wrong response?
The way I understand it, there are three possibilities of how their, and our, hearts can be inclined:
- We can be inclined toward neither good nor evil, but have free will to choose either
- We can be inclined toward evil
- We can be inclined toward good
The people in Noah's day were inclined toward evil because of their sinful nature and because they continued to reinforce that sinful nature by adding sin to sin to sin. They conditioned themselves to react to each new situation with evil thoughts.
The third direction in which a human heart can be inclined is toward the good. When we accept salvation and allow God's Spirit to cleanse our hearts, we are once again given the opportunity to choose either the good or the evil, just as Adam and Eve could freely choose. If we listen again to our sinful nature, we will fall away from Christ, or at least not grow in Him. But if we listen to the Spirit, by practice and repetition, we will train our hearts to incline toward the good. This is spiritual growth. This is discipleship. This is allowing God to give us a new heart.
So, where are you on that scale of heart inclination? How do you respond to things? What is your knee-jerk reaction? Is it toward evil or is it toward good? Maybe you're somewhere in the middle. Gauge your thoughts and ask God to continually cleanse and train your heart toward Him. The people of Noah's day would not listen to God's Spirit when He tried to contend with their spirits. Let us not be as hard-hearted when God speaks to us.