To be Faithless is to be in Unbelief.   

To be Perverse is to be controlled by Opinion and Tradition rather than the Truth.   

In the situation where the Disciples were unable to cast the Demon out of the the young man, their limitation was in themselves and not in God.   

When they heard the father's description of his son's condition and then saw the young man go into one of his fits two things happened in their Minds and Hearts:

1) They allowed the severity and longevity of the condition to cause them to Doubt.   
They became Faithless.   

2) They remembered a long history of similar situations which persisted throughout the life of the afflicted.   
They became Perverse, forgetting what Jesus said and being convinced by Tradition.   

The same Disciples who had Healed and Exorcised in Victory on previous occassions, took their eyes off of Jesus and consider only the Circumstances.   
They had operated from the Weakness and Unbelief of the generation they lived in.   

The cure for their condition was Fasting and Prayer.   They needed to more rigorously Receive the Words of Christ (see HeBeReDo).