Charity - giving to the poor - is an essential part of Christian morality: in the frightening parable of the sheep and the goats it seems to be the point on which everything turns. I do not believe one can settle how much we ought to give. I am afraid that the only safe rule is to give more than we can spare. In other words, if our expenditure on comforts, luxuries, amusements, etc., is up to the standard common among those with the same income as our own, we are probably giving away too little. If our charities do not at all pinch or hamper us, I should say they are too small. There ought to be things we should like to do and cannot do because our charities expenditure excludes them. I am speaking now of 'charities' in the common way. Particular cases of distress among your own relatives, friends, neighbours or employees, which God, as it were, forces upon your notice, may demand much more: even to the crippling and endangering of your own position. For many of us the great obstacle to charity lies not in our luxurious living or desire for more money, but in our fear - fear of insecurity. This must often be recognised as a temptation. Sometimes our pride also hinders our charity; we are tempted to spend more than we ought on the showy forms of generosity (tipping, hospitality) and less than we ought on those who really need our help.
CS Lewis, Mere Christianity, p. 86-87