A: But do we have to see things in order to believe in them? Did anyone see atoms or subatomic particles, etc.? Our senses are very limited and unreliable. To claim that something does not exist simply because I have not seen it is a childish logic. I have not seen you - therefore you don't exist? I may hear about you, see your pictures, talk with reliable persons who know you, etc. and thus I can be pretty sure there is such a person as you. If I set on a journey and fulfill the necessary conditions to reach your place (visa, tickets, maps, etc.), I may meet you in person. All of this can be also applied to God. In His case the 'journey' means the spiritual process of bhakti-yoga.
Although seeing God is not the primary goal of Vaisnavas (they prefer to serve God, Krsna, in such a way that He will want to see _them_) it is possible on the level of pure bhakti-yoga, as its by-product. The main way is through chanting Krsna mantras, especially the Hare Krsna mahamantra. This is the yajna and dharma for this age. However, it is important to remember that it is up to Krsna if He reveals Himself to us; we have no power other than our love.
But if there is God, why we don't see Him? The Vedas answer that because living beings in this material world don't want to see Him He makes Himself invisible. Why they don't want to see Him? Because this world is like a playground for the rebellious beings who want to be independent of God. Therefore He mercifully provides us with this facility. At the same time He cares for us in various ways and reminds us that this is not our real home because we suffer here (this world is also compared to jail) and that we should return home, to Him.
Another important thing is to know who God is. Otherwise we may see someone and don't recognize Him as God. Several times in past (as Sri Caitanya Mahaprabhu, Nityananda Prabhu, Lord Jesus) He was not recognized by majority of people during His appearance on Earth. Someone may also pose as God in order to cheat us.
(Path of Perfection, chapter three: Learning How to See God):
Comments
Many have seen the lord in reality.
As you said it is only by His mercy that He gives darshan.