What was Johannes Brahms religion?

Answer 1

Brahms was brought up a Lutheran protestant.

Apparently, he read The Bible throughout his life, but early on distanced himself from Christian doctrine. He regularly used the Bible as a source of powerful poetry, e. g. in his choral motets op. 29, op. 47, and op. 110, as well as in his "Four Serious Songs" and in the "German Requiem".

However, when reading the texts chosen for the latter carefully, you will notice that all the texts are about the human condition in general, about grief, suffering, the vanity of human existence, and about the promise of consolation. All direct connections to Christ, to the resurrection, or to sin and forgiving are carefully avoided. In a letter written short before the first performance, Brahms all but renounced Christian doctrine explicitly when referring to John 3:16 ("For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life.") and saying that he would do rather without that line.

This was noticed by Brahms' contemporaries immediately. When the German Requiem was first performed in Bremen cathedral, on Good Friday, 1868, the conductor suggested to insert movements by Bach and Handel to make up for the lack of a statement of specifically Christian belief.