The text does not say they could not hear and understand but they were stiffnecked - stubborn in not believing what their ears heard Stephen preached.
The questions that remain: why were they "cut to the heart" and gnash on Stephen with their teeth if they could not/did not understand the words Stephen spoke to them? And why would Stephen even speak to them if he knew that could not even understand what he was preaching anyway? And what's the point in the great commission (Mt 28:19,20; Mk 16:15,16) in preaching the gospel to every creature/all nations if they cannot understand what is being preached?
The truth is not all have been given ears to hear, but like the Lord told Ezekiel, whether or not the people will hear, you still need to speak His words, even if it's only for a witness against. Being
uncircumcised in heart and ears means they are yet
carnal, and yet without understanding. They gnashed at Stephen because the children of the
flesh(darkness) hate the children of the Spirit, or Light.
They hardened their hearts to the message, they became stiff-necked and rebellious resisting the Spirit, same is shown here...
Matthew 13:14-15[SUP]
14 [/SUP]And in them is fulfilled the prophecy of Esaias, which saith, By hearing ye shall hear,
and shall not understand; and seeing ye shall see,
and shall not perceive:
[SUP]
15 [/SUP]For this people's heart is waxed gross, and their ears are dull of hearing, and their eyes they have closed; lest at any time they should see with their eyes and hear with their ears,
and should understand with their heart, and should be converted, and I should heal them.