Question

What advice does machiavelli give princes regarding religion?

Answer 1

In fact, he never said "the end justify means"  , but he refers to it in his book "The Prince". He used to say "For although the act condemns the doer, the end may justify him".

He implied that "the means" matter, and "the end " don't always justiy them. Nevertheless, it is worth accepting all the ramifications of "unjustifiable means".

Answer 2

The advice that Machiavelli gave the princess regarding the princess was the end justify the means.
And a ruler can do anything that will promote the power of the states, including using religion.