The Eternal Father speaks to Catherine about charity:

It is like a vessel that you fill at the fountain. If you take it out of the fountain to drink, the vessel is soon empty. But if you hold your vessel in the fountain while you drink, it will not get empty. Indeed it will always be full. So the love of your neighbour is meant to be drunk in Me without any self-interest.

I ask you to love Me with the same love with which I love you. But for Me you cannot do this, for I loved you without being loved. Whatever love you have for Me you owe Me, so you love Me, not gratuitously but out of duty, while I love you not out of duty, but gratuitously. So you cannot give me the kind of love I ask of you. This is why I have put you among your neighbours: so that you can do for them what you cannot do for Me – that is, love them wihtout any concern for thanks and without looking for any profit for yourself. And whatever you do for them I will consider done for me.

So your love should be sincere: You should love your neighbours with the same love with which you love Me. Do you know how you can tell when your spiritual love is not perfect? If you are distressed when it seems that those you love are not returning your love and not loving you as much as you think you love them. Or if you are distressed when it seems to you that you are being deprived of their company or comfort or that they love someone else more than you.

From these and from many other things you should be able to tell if your love for Me and for your neighbours is still imperfect and that you have been drinking from your vessel outside of the fountain, even though your love was drawn from Me. But it is becasue your love for Me is imperfect that you show it so imperfectly to those you love with a spiritual love.

From the Dialogue of St Catherine of Siena – Chapter64