Per p. 191 of the Player's Handbook, you can't move through a hostile creature's space unless they're two sizes larger or smaller than you. You can move through a nonhostile creature's space but can't willingly stop there. Either way, another creature's space is considered difficult terrain.
The guardian of faith spell creates a "Large spectral guardian" in "an unoccupied space of your choice". My question is, should the guardian be regarded as a creature for purposes of moving through its space, or is it an impassable barrier? Nowhere in the spell description is it described as a creature, nor is it given an AC and hit points that would allow it to be attacked as one. It doesn't get opportunity attacks against creatures that move away from it. On the other hand, it's explicitly described as Large, and I can think of no other reason why that information would be relevant. Is there an official answer to this, or is it up to the GM?