Certain things we may not be able to see but we may believe their existence. We have not seen God. But many people believe the existence of God. But some may not believe. As expressed by the author we can try to understand the issues which we have not seen and then come to an understanding. But simply not believing as you have not seen without thinking about the issue is not justifiable.
A saint in the Ashram of Shirdi Baba at Pune was asked by a visitor whether he have seen Baba in person. He, the saint replied 'No'. Then the visitor asked how he believed the Baba. The saint asked him as a reply whether he was having seen dogs. He replied that he himself have a dog in his house. The saint replied that," If a dog in some place have seen a thief it started barking and by hearing its barking the dogs of other areas are started barking though the other dogs have not seen the thief. Similar to that my forepersons have seen Baba and wrote about Him and by believing their words we, other people do believe.