Many Lutherans sing the beloved hymn, O Sacred Head Now Wounded, (LSB 450) during Lent and especially on Good Friday. Paul Gerhardt wrote the German hymn, O Haupt Voll Blut und Wunden, upon which this hymn is based in the 17th century. 
Johann Sebastian Bach later incorporated elements of this hymn into his St Matthew Passion. However, one section of a late medieval Latin devotional poem to the crucified Christ inspired Gerhardt’s German text. German, Latin, and English Texts
Traditionally attributed to the famous Cistercian abbot and preacher, Bernard of Clairvaux (d. 1153), this hymn reflects the growing significance of graphic depictions of the suffering Christ in late medieval devotion. While Bernard’s writings may have inspired the spirituality associated with this poem, he certainly did not write it. He often described the Passion of Christ as God’s redemptive action and as an example for believers to follow. Bernard’s Meditation on the Lord’s Passion
However, a Cistercian abbot, Nicolas Salicetus, did publish the very long devotional poem with extended prayers to Christ’s feet, knees, side, breast, and face. He claimed Bernard prayed this before a crucifix.* The Latin text from which Gerhardt drew his German hymn derived from Salicetus’ text. Since Martin Luther and the Lutheran Confessions cite much of Bernard’s writings approvingly, it makes sense that a later Lutheran hymn writer would use a devotion to Christ’s suffering connected to the Cistercians.
In 1519 Dr. Martin Luther began to apply his theology of justification by faith alone to Christian piety and sacramental theology. In this year, he published numerous devotional works and three short treatises on Baptism, the Lord’s Supper, and Penance in German. On April 5 Luther sent a copy of a sermon on the Passion of Christ to Georg Spalatin, who served as secretary and chaplain to Frederick the Wise, Elector of Saxony. Luther’s sermon on the Passion was published numerous times over the next five years and a version of it appeared in Luther’s Church Postil as a sermon for Good Friday. In this work he set forth a guide in the proper way to meditate on the Lord’s Passion.**
Here Martin Luther reinterprets an established medieval practice based upon his emerging theology of the cross and doctrine of justification. First, Luther explains that Christians should not meditate on Christ’s suffering to become angry with the Jews or imagine that crosses or images of Christ ward off evil or natural disasters. Second, Christians should not pity Christ as an innocent sufferer as the women along the road to Calvary. Instead Luther writes:
They contemplate Christ’s passion aright who view it with terror-stricken heart and a despairing conscience. This terror must be felt as you witness the stern wrath and the unchanging earnestness with which God looks upon sin and sinners, so much so that he was unwilling to release sinners even for his only and dearest Son without his payment of the severest penalty for them.***
He explains that believers must realize that their sins tortured Christ by the nails piercing his feet and hands. This image of the crucified Christ functions like a mirror to reveal utter human sinfulness. Luther then describes in some detail how Christ’s bodily and spiritual suffering should compel us to repentance. In fact, he compares it to the death of the Old Adam in baptism.
According to Luther, Christians must not place their confidence in pilgrimages or contrived ‘good works’ of penance. True repentance sees Christ’s wounds and suffering as the place for sins. The more sin torments one’s conscience, the more he should trust that Christ took them upon Himself. After citing Isaiah 53:6, I Peter 2:24, and II Cor. 5:21, Luther concluded:
Sin cannot remain on Christ, since it is swallowed up by his resurrection. Now you see no wounds, no pain in him, and no sign of sin. Thus St. Paul declares that ‘Christ died for our sin and rose for our justification’ [Rom. 4:25]. That is to say, in his suffering Christ makes our sin known and thus destroys it, but through his resurrection he justifies us from all sin, if we believe this.****
Once someone meditates on Christ’s Passion and trusts in His true love, he may then follow Christ as an example and pattern for life. In this way, the Christian can bear sickness, temptation, pride, and fleshly lusts. Thereby, believers begin to live as true Christians.
*Sheryl Frances Chen, OCSO, “Bernard’s Prayer Before the Crucifix that Embraced Him: Cistercians and Devotion to the Wounds of Christ,” Cistercian Studies Quarterly 29 (1994): 23-40.
**Luther’s Works (LW) 42:5-6 (intro), 7-14; Church Postil II in LW 76: 425-432.
***LW 42:9.
****LW 42:12-13