My question is this. Why do you believe in the Christian God and not a Hindu? or from an African tribe? Why is your version of God better?
Well, let's make a definition of God. It will sort the things out. God is the source of everything, that's the first definition. It's from Vedanta (janmādy asya yataḥ). The second definition is from Vishnu-Purana (6.5.47). Parasara Muni, a great sage and the father of Vyasadeva, who compiled all the Vedic literatures, gave the following definition of God:
aisvaryasya samagrasya viryasya yasasah sriyah
jnana-vairagyayos caiva sannam bhaga itingana
Bhagavan, the Supreme Personality of Godhead, is the one who is full in six opulences — who has full strength, fame, wealth, knowledge, beauty, and renunciation.
So when I'm saying I believe in God I mean that kind of God. I mean the very specific God, not just God. And from the first definition we can clearly see, that such God which is the source of everything (!) is only one. The key word is everything.
Another definition of God is "asama urdhva", which means there is no equal to Him (asama), and nobody is greater than Him (urdhva). So if we use this definitions we can't see many Gods.