Martin Luther wrote this hymn as a declaration of his faith! Having faced trials, excommunication, sickness, depression, spiritual warfare, and death, he knew that God, his Mighty Fortress, would be his bulwark never failing. .
In Rome, Martin saw with his own eyes the end result of all the money being made by the church and the sale of indulgences and the exhibition of relics. He also had read the Bible and never found any passage proving that one could purchase forgiveness wit.
If you’re lacking joy or peace or both, try trusting God and putting your hope in Him and Him alone (Proverbs 3:5-6; Psalm 42:5). He is “the God of hope,” so run to Him when you’re feeling hopeless..