So many miracles - This does not refer to any miracles performed on this occasion, but to all his miracles performed in view of the nation, in healing the sick, opening the eyes of the blind, raising the dead, etc. John here gives the summary or the result of all his work undeniable proof of his being the Messiah, yet the nation did not believe on him.
Before them - Before the Jewish nation. Not in the presence of the people whom he was then addressing, but before the Jewish people.
They believed not - The Jewish nation did not believe as a nation, but rejected him.