These words come from Psalm 23. It is actually one of the most beloved and well-known Psalms of all time. The psalm basically makes two points. First, God is a good shepherd, who leads his people. Second, in light of the first point, God's people have no need to fear, even if they walk in the valley of the shadow of death. God is always a present reality and he will defend his people and bring them into glory.
These words are very appropriate for the work, A Man for All Seasons, because in the end Sir Thomas More dies for his faith as he opposes King Henry. In other words, Sir Thomas More was able to be faithful to his conscience and his principles even if meant death, because he knew that God was with him.