About finite or infinite universe, remember time is a factor. Planets and whole systems collide, break apart, create new stars and planets. So you don't really need an infinite amount of mass to have an infinite universe, or rather, an infinite number of possibilities when it comes to evolving sentient life.

Then, when it comes to star-travelling species, time becomes less of a factor. For them to be able to reach us, you almost will have to strike time from the equation. They might have been looking for soemthing like us for billions of years by now! As has been mentioned, time is a big factor when it comes to civilazations. But then, if a society evolved past its planet, or star system, there would be very little stopping them.

When it comes to hostility, it's very hard to guess. Maybe we are the first race tehy encounter? maybe they encountered millions of races before?

Maybe they even fought an interstellar war or two allready and are now very centered on never letting it happen again.

However, the fact we aren't whiped out IF any other species knows about us, says a lot about their hostile level.

About them being something like us... We just need to have a look at our own world to see the possibilities of life he could be more sentient.

Ants, as an example... Hundreds of drones for every queen, the queen being the only sentient life... If you add genetical engineering to ants allready impressive genetic abilities in creating various offspring, we could have a race with very specialised individuals (if individuals would be the right word).

But to get the church into this, could the church cope with a handfull of ant-alikes suddenly visiting us? Or would it be the end of religion as we know it?