I'm bumping to put in my two cents on this.
It's been historically proven that two religions can grow up, side by side, for centuries, and then one suddenly, violently, subsumes the other. Just because the Solari now are a group of people dedicated to protection and leadership, doesn't mean they were in the past. It's quite plain to see that the Lunari, as a whole, were destroyed. Diana finding the Chosen's equipment could then either be called a coincidence or the calling of Lunari to bring a new champion to its fold.
I propose this, then: Formerly, the two religions were side by side, working together. Likely, the Solari were a more public and open face (leaders, paladins, guardians), while Lunari was more secretive--perhaps assassins working to kill those who tried to disrupt the worship of the Sun and Moon. When the Solari realized the power they held, they could have decided, to hell with the little guy--we can do this on our own.
And so, with greater numbers, the Solari brought down their force and their current Champion, and destroyed the Lunari, their Champion, and took over their duties.
Now, Diana finds the armor and sword, finds Lunari. She tries to bring this to the Elders of the Solari. They may be nice now, but they've doubtlessly received warnings from their forerunners--after all, history is written by the victors. They call Diana a heretic, and try having her killed. Diana responds in kind, and now, the whole thing is replaying itself in the League of Legends. Likely, Leona and Diana will eventually come to a head and fight, and one of two things will happen:
One will destroy the other, and the winner begins rebuilding their worship and damning the other.
Or, the two will forgive each other for their sins, and the Lunari and Solari can once again come into balance and try to actively help the other.