To return briefly to our friend, Machiavelli, the father of modern political philosophy, I'd like to summarize his beef with Christianity.
He saw all around him, in almost-Renaissance Italy the entanglement of Church and state. It was a mess. The problem, as he saw it, was that the Church is a universal institution (i.e., open to everyone) while the state, or principality, is a local institution (i.e., exclusive to a certain group of people, the citizens). The result is a group of citizens with two allegiences: one to the local polity, the other to the universal Church. Machiavelli saw this conflict as a source of political unrest and a threat to the stability of the state.
Two solutions have arisen in modern politics, as I 've mentioned before. The first is a nationalization of the Church, as happened in England under Henry VIII and his progeny. The second is the "separation of church and state" as in the USA.
Machiavelli's second and perhaps bigger beef with Christianity was that it introduced the problem of "the other world." The highest good for human beings is, in the Christian faith, heaven. This, cries Machiavelli, "has rendered the world weak." Christian citizens do not care enough about saving this world, for all their hope is in the next; they are bad citizens.
Machiavelli's solution was to work within a sort of hardline, secular realism: the entire focus of his Prince is on solving the problems posed to princes in this life. He does not hold the ruler or the ruled, for that matter, to any standard outside of this world. In this way, he works to remove the problem of heaven from the problems of politics and found a city based on human reason alone.