The Holy knights could be Gargoyles a fantasy / horror Monster, which are Statues physically coming to life. I came to this Idea because people kept referring to this panel with said Statues.
Gargoyles are also often guardians of places such as cathedrals or castles
From the Silhouettes in Chapter 1083 we know, that at least one of them wields a Sword.
People on Twitter also kept bringing up the Cover of Chapter 699 they call it a foreshadowing by Oda to said Knights.
Interestingly enough, Luffy has a Sword while Sanji has a Spear/Lance like weapon, maybe it tells us that the 9 Knights are different type of Weapon specialists? Usopp holding a Bow could be a little stretch though, because i can't see anything that resembles a bow in the hands of the Statues above, but who knows. Having a strong Bow User in One Piece that Usopp could face, would be cool.
Not like these weaklings below
Probably won't come true, but i thought i throw it in here.