I myself have pondered these questions, although the lack of major heavenly fireworks has never caused me to doubt (not that I haven't doubted - I have... a lot). In the words Judas sings in the rock opera, "Jesus Christ Superstar," - "If you'd come today, you could have reached a whole nation. Israel in 4BC had no mass communication... don't get me wrong... I only wanna know..."
The truth is, God has revealed himself in big, huge ways numerous times throughout history. As if the coming of God Himself as a man in the form of Jesus wasn't enough - there have been many signs. From Old Testament times straight through until today, people have witnessed amazing things. Miracles, visions, apparitions, signs. I was reading a discussion of this week's readings at The Sacred Page today which I found fascinating - talking about some of those "Big Events" when God has appeared to large groups of people. From Biblical times to modern, those events, while noteworthy and important for many who witnessed them, have never been very effective at actually changing people's hearts and turning them toward God. In short, coming out with trumpets and fanfare, earthquakes and a booming voice from the heavens, hasn't ever worked very well to instill faith in God. As John Bergsma writes, God appeared at Sinai (Exodus 19) and only 40 days later, the people were worshipping a false God again. Just 40 days! He showed his power in the time of Elijah at Mount Carmel and the next day Elijah is forced to flee for his life from the pagan Queen. In modern times, there have been signs, but they don't result in mass conversions. They do little to change the human heart.
Have you come to believe because you have seen me? Blessed are those who have not seen and have believed. ~John 20:29
How does God reach out to us and change our hearts? If not in grand displays of power, how? More often than not, He whispers. His voice is soft and gentle and we have to learn to quiet ourselves in order to hear it. In the quiet, in the silent place of our hearts, God speaks.
It may not always be easy to hear, but that is our failing, not His. It is easy to be consumed with our own issues and fears, our problems and obsessions. All that noise, the rattling and banging around in our heads. He calls to us gently, through all the racket and if we slow down long enough, we just might hear Him.