Proslogium and Monologium/Monologium/Chapter76

Chapter LXXVI
We should believe in Father and Son and in their Spirit equally, and in each separately, and in the three at once.

We should believe, then, equally in the Father and in the Son and in their Spirit, and in each separately, and in the three at once, since the Father separately, and the Son separately, and their Spirit separately is the supreme Being, and at once Father and Son with their Spirit are one and the same supreme Being, in which alone every man ought to believe; because it is the sole end which in every thought and act he ought to strive for. Hence, it is manifest that as none is able to strive for that Being, except he believe in it; so to believe it avails none, except he strive for it.