You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Martin_Luther

Context:
In this work, one of his most emphatic statements on faith, he argued that every good work designed to attract God's favor is a sin. All humans are sinners by nature, he explained, and God's grace (which cannot be earned) alone can make them just. On 1 August 1521, Luther wrote to Melanchthon on the same theme: "Be a sinner, and let your sins be strong, but let your trust in Christ be stronger, and rejoice in Christ who is the victor over sin, death, and the world. We will commit sins while we are here, for this life is not a place where justice resides."

Question:
What did Luther explain about acquiring God's grace?

cannot be earned