This is from Luther's commentary on Galatians 6:6. Although it was written with particular application to the material excesses of medieval Roman Catholicism, the general application--of financially supporting those who preach and teach in our local congregations--is still as relevant as when Paul penned his letter to the Christians at Galatia:
Verse 6. Let him that is taught in the word communicate unto him that teacheth in all good things.
Now the Apostle also addresses the hearers of the Word requesting them to bestow "all good things" upon those who have taught them the Gospel. I have often wondered why all the apostles reiterated this request with such embarrassing frequency. In the papacy I saw the people give generously for the erection and maintenance of luxurious church buildings and for the sustenance of men appointed to the idolatrous service of Rome. I saw bishops and priests grow rich until they possessed the choicest real estate. I thought then that Paul's admonitions were overdone. I thought he should have requested the people to curtail their contributions. I saw how the generosity of the people of the Church was encouraging covetousness on the part of the clergy. I know better now.
As often as I read the admonitions of the Apostle to the effect that the churches should support their pastors and raise funds for the relief of impoverished Christians I am half ashamed to think that the great Apostle Paul had to touch upon this subject so frequently. In writing to the Corinthians he needed two chapters to impress this matter upon them. I would not want to discredit Wittenberg as Paul discredited the Corinthians by urging them at such length to contribute to the relief of the poor. It seems to be a by-product of the Gospel that nobody wants to contribute to the maintenance of the Gospel ministry. When the doctrine of the devil is preached people are prodigal in their willing support of those who deceive them.
We have come to understand why it is so necessary to repeat the admonition of this verse. When Satan cannot suppress the preaching of the Gospel by force he tries to accomplish his purpose by striking the ministers of the Gospel with poverty. He curtails their income to such an extent that they are forced out of the ministry because they cannot live by the Gospel. Without ministers to proclaim the Word of God the people go wild like savage beasts.
Paul's admonition that the hearers of the Gospel share all good things with their pastors and teachers is certainly in order. To the Corinthians he wrote: "If we have sown unto you spiritual things is it a great thing if we shall reap your carnal things?" (I Cor. 9:11.) In the old days when the Pope reigned supreme everybody paid plenty for masses. The begging friars brought in their share. Commercial priests counted the daily offerings. From these extortions our countrymen are now delivered by the Gospel. You would think they would be grateful for their emancipation and give generously for the support of the ministry of the Gospel and the relief of impoverished Christians. Instead, they rob Christ. When the members of a Christian congregation permit their pastor to struggle along in penury, they are worse than heathen.
Before very long they are going to suffer for their ingratitude. They will lose their temporal and spiritual possessions. This sin merits the severest punishment. The reason why the churches of Galatia, Corinth, and other places were troubled by false apostles was this, that they had so little regard for their faithful ministers. You cannot refuse to give God a penny who gives you all good things, even life eternal, and turn around and give the devil, the giver of all evil and death eternal, pieces of gold, and not be punished for it.
The words "in all good things" are not to be understood to mean that people are to give all they have to their ministers, but that they should support them liberally and give them enough to live well.
(Source, with bibliographic information at the bottom of the page.)