One way God provided proof of His existence was through miracles. Both the Old Testament and New Testament recorded miracles. The Bible records the miracles for the benefit of future generations.
But some people still refused to turn to God in spite of having witnessed the miracles. Some people will not turn to God even if they witnessed a miracle because of their hardened heart.
Just as God used the prophets and apostles to share the Good News about God, God still chooses to work through people to make Himself known to us.