For God requires above all that we worship Him in spirit and in faith. In observing the Law for the purpose of obtaining righteousness without faith in Christ these law-workers go smack against the Law and against God. They deny the righteousness of God, His mercy, and His promises.
 Martin Luther, Commentary on the Epistle to the Galatians (1535). copy citation

add
Author Martin Luther
Source Commentary on the Epistle to the Galatians
Topic workers faith
Date 1535
Language English
Reference
Note Translated by Theodore Graebner
Weblink http://www.gutenberg.org/files/1549/1549-h/1549-h.htm

Context

“They call a "doer" of the Law one who performs the Law in its literal sense. This is not to "do" the Law. This is to sin. When our opponents go about to perform the Law they sin against the first, the second, and the third commandments, in fact they sin against the whole Law. For God requires above all that we worship Him in spirit and in faith. In observing the Law for the purpose of obtaining righteousness without faith in Christ these law-workers go smack against the Law and against God. They deny the righteousness of God, His mercy, and His promises. They deny Christ and all His benefits.
In their ignorance of the true purpose of the Law the exponents of the Law abuse the Law, as Paul says, Romans 10:3, "For they, being ignorant of God's righteousness, and going about to establish their own righteousness, have not submitted themselves unto the righteousness of God."
source