It's just as what Jesus had said unto Thomas,"Jesus saith unto him, Thomas, because thou hast seen me, thou hast believed: blessed are they that have not seen, and yet have believed." John 20:29
It is also sad that even if miracles were to be performed and observed in front of their 2 very eyes, atheists and agnostics will still maintain their spiritual blindness.
What is even worse is that there are those who follow occultic practices and such as tarot cards and palm reading and so-called psychics who can conjure up dead family members and yet fail to acknowledge the Divine power of Christ and the reality of His existence in time past.
God Bless!