The giving of offerings and alms becomes an idolatrous work when it is seen as "doing a good work deserving of thanks" instead of the sanctified work of the Holy Spirit as Luther maintains in this excerpt from vol. 21 of the American Edition of his writings.
You see what a shameful distortion of good works this is, and how common a fault it is throughout the world that nobody does anything good without such motivation.
The world cannot get rid of this crazy idea, and it cannot endure or overcome ingratitude. This is also where the monks came from. They ran away into the desert because they were so weak that they could not stand the thought of being in the world, helping everyone and doing good, and then of being rewarded for this with nothing but contempt and malice, insults and ingratitude.
But what sort of devil is it that tells you to do these works on the ground that by them you will earn the popularity of the world, which is uncertain and can quickly collapse and change, rather than to look for a better motivation, namely, God? For then they can never be lost, and He will richly reward you, both here and hereafter.
Therefore it serves you right for being such a scoundrel and having no other purpose than to be worshiped by the people and to make a god of yourself! So He can very easily let the world and the devil take care of you. They will take your divinity away from you and throw it into the filth, which is exactly where it belongs. Since you have the effrontery to seat yourself on God’s throne and to lay claim to the glory that belongs only to Him, it is fitting for Him to cast you down again so that complete disgrace is all the thanks you get in place of your stolen honor.
It is really disgraceful, the way the world carries on: it may be pious or it may be wicked, but either way it is worthless. Either it tries openly to be a devil with its wicked works, or it tries to be God Himself with its good works. And both of these are intolerable. Therefore no one can do a truly good work unless he is a Christian. If he does it as a man, then he is not doing it for the glory of God, but for his own glory and advantage. On the other hand, if he claims that it is for the glory of God, that is a lie that smells to high heaven.
For this reason Christ now intends to teach about the right way of giving alms, and He says: “When you give alms, do not have a trumpet sounded or a loud shout raised before you, so that a whole city has to know about it and talk about it. (So it is among us, when a charitable contribution is being made and all the bells have to be rung.) But when you give alms, do it in such a way that your left hand does not know what your right hand is doing.”
This is the self-same thing that St. Paul says in Romans 12:8 and elsewhere (2 Cor. 9:7): “He who contributes, let him do it with singleness.” But “contributing with singleness” means that by making the contribution one does not seek his own glory or popularity, gratitude or reward, and is not concerned about whether any human being is grateful or not. But he contributes freely what he wants to contribute, just the way God grants His gifts every day and causes His sun to shine, regardless of the thankful or the unthankful, just as if He did not see anybody. A heart is truly single in its motivation if it neither seeks nor desires nor looks at anything except the will of God and the glory of God.
Such sincere almsgiving is not to be found among worldly people; their contributing is the kind that gives with the right hand and grasps with the left hand. This is what is called “givers, takers,” a phrase the children use to taunt each other, the kind of giving that expects to acquire ten times as much, contributing a drop of water and then getting a cask of wine back.
Worldly people make their contributions in order to have a glory that is immensely greater than all the money and property in the world. They want to buy you for a trifle and to hold you as their perpetual prisoner — your body and your life and everything you have — along with God Himself. For that reason Christ says: “When you are giving alms with your right hand, be careful that you do not try to take more with your left hand. Hold it behind your back, and do not let it know anything about what is going on.” Then you may be said to be “contributing with singleness,” not to be taking or to be giving in such a way as to make the other person owe you ten times as much as you gave him, or to make him celebrate and adore you as an idol.
That is the way our young squires proceed now. If they have rendered a service to someone with a gulden or two, they imagine that they have bought him and that he is so deeply obligated to them that he has to call all their words and actions golden and dare not say a word to them except what they would like to hear.