It is like my telling my employer that I am the last CEO, or COO, or MD, etc. Say if Madmohan oops Manmohan Singh claims that he is the last PM of India and that there can be nor more PM what will happen to India.
I for one do not understand this last business. Why do all these people say they are last. So that their name remains ever and ever in history.
In other words, you do not leave any choice to God. Suppose down the line God wants to send some more messengers, this Jesus and Mohammed have sealed the options.
That way Hindu religion is the best which says that as and when God requires based on the situation in the world more Avataars will take birth.
