For example,
vishu - protects good - have devotees who are peace loving
shiva - destroys bad - have devotees who are anxious
Another example,
in high court there is only one judge,here the justice may be correct or wrong.
But,in supreme court,there are three judges, here the justice will be correct as the desicion should be equivalent with the three judges.
Hence,god is to be one.As no one seen the god and a religion is just a society of people with different thoughts,we may be according to our religion says.But to those who are hindu's I request them that please do not yell at hindu religion.
If any hindu do not like hindu religion or it's principles please be quiet or try to know what great knowkedge is in it.