Do we truly know what the Lord thinks of unbelief arising in the heart of a follower of Christ? Look at the terminology the Bible uses,
“Beware, brethren, lest there be in any of you an evil heart of unbelief in departing from the living God” (Hebrews 3:12).
First, the writer warned of a danger, and then he revealed the danger, when we allow unbelief to creep into our hearts, which he called evil. It is evil because it will cause us to depart from the living God.
Christians are people who have already stated that they believe, but then to turn to unbelief, means they believed a lie the first time, which means they charge God with lying!
This does not address those who never have believed, and are weighing the evidence, but this addresses those who have already embraced the Lord.