Acts 28:27 (NKJV)
For the hearts of this people have grown dull. Their ears are hard of hearing, And their eyes they have closed, Lest they should see with their eyes and hear with their ears, Lest they should understand with their hearts and turn, So that I should heal them.
The first thing Paul does after arriving in Rome is to call a meeting of the leaders of the Jews there. This was always his pattern wherever he went. He would first visit the Jewish synagogue and then branch out to the gentiles in the area. Some believed and some rejected. He quotes from Isaiah to show the process of coming to faith in Christ: "See with their eyes," (observe in a curious way), "hear with their ears," (stay around long enough to listen and consider), "understand with their hearts," (believe the truth of the word), "and turn" (repent of their ways and turn toward Jesus), "So that I should heal them," (be made new by God). This is the process of salvation. It requires reason, understanding, repentance and faith.
Why would anyone reject God through unbelief? I believe it all boils down to pride. It takes a certain amount of humility to realize your incapacity to save yourself and understand your need for a savior.
James 4:6 (NKJV)
Therefore He says:"God resists the proud,but gives grace to the humble."