I'm not familiar with the religion improvement, what is that and what does it do?
More Prophets are used for spreading, in particular that the prophet knocks out all other religions before spreading his own. The massively leveraged use of a Prophet is to hit an enemy's holy city as soon as its religion is founded. This is an enormous setback to even get out a missionary after that; the victim has to wait for the internal holy city pressure to swing the city back, wich takes dozens of turns. Meanwhile your faith's pressure converts all their cities. Read this, starting about halfway down at the 1 AD screenshot. http://dos486.com/civ4/civ5gkocc/
I very much think at least one prophet doing that will more than pay back his cost via Pilgrimage, so take at least one at 300 faith before Hagia.
The other thing you need a Prophet for is undoing an AI's prophet spreads to you, which do the same thing (though less well leveraged) and take forever to recoup by pressure or inquisitor unless reversed by your own prophet. This will happen later though.
More Prophets are used for spreading, in particular that the prophet knocks out all other religions before spreading his own. The massively leveraged use of a Prophet is to hit an enemy's holy city as soon as its religion is founded. This is an enormous setback to even get out a missionary after that; the victim has to wait for the internal holy city pressure to swing the city back, wich takes dozens of turns. Meanwhile your faith's pressure converts all their cities. Read this, starting about halfway down at the 1 AD screenshot. http://dos486.com/civ4/civ5gkocc/
I very much think at least one prophet doing that will more than pay back his cost via Pilgrimage, so take at least one at 300 faith before Hagia.
The other thing you need a Prophet for is undoing an AI's prophet spreads to you, which do the same thing (though less well leveraged) and take forever to recoup by pressure or inquisitor unless reversed by your own prophet. This will happen later though.