From “The Interior Castle” by Teresa of Ávila
There are only two duties which our Lord requires of us, namely, the love of God and the love of our neighbour; these are the objects we must labour for; by observing these laws perfectly, we do his will, and consequently, we shall be united with him. But, as I have said, how far are we from observing these two duties as we ought to do towards so great a God! May his Majesty grant us grace, in order that we may deserve to arrive at this state; and this is in our power if we wish.
In my opinion, the surest sign for discovering whether we observe these two duties is the love of our neighbour; since we cannot know whether we love God, though we may have strong proofs of it; but they can be more easily discovered respecting the love of our neighbour. And be assured, that the further you advance in that love. the more will you advance in the love of God likewise; for the affection which his Majesty has for us is so great, that as a return for the love we show our neighbour, he will make that love go on increasing which we have for himself: of this I have no doubt.
It is very important for us diligently to observe how we proceed in this matter; for if we endeavour perfectly to acquire this love of our neighbour, we shall have done everything; because, as our nature is corrupt and evil, unless it comes from the root (which is the love of God), we shall never perfectly possess the love of our neighbour.