The doctrine in Hymns - "A Mighty Fortress is our God" (verse 3)

Verse 3, #1: And though this world, with devils filled, should threaten to undo us,

The love of the temporal things of this world is promoted and used by Satan to harm and destroy mankind. Satan uses these worldly desires to destroy and harm as many people as he can. The sin and evil that results from the lust for power, money, success, sex, fame, etc. can destroy the physical body, and also destroy the soul. This threat is real and one that Christians should be on guard against.

James 4:4 You adulterous people! Do you not know that friendship with the world is enmity with God? Therefore whoever wishes to be a friend of the world makes himself an enemy of God.
Luke 22:31-32 “Simon, Simon, behold, Satan demanded to have you, that he might sift you like wheat, (32) but I have prayed for you that your faith may not fail. And when you have turned again, strengthen your brothers.”

Verse 3, #2: We will not fear, for God hath willed His truth to triumph through us:

God has ordained that His truth will win the ultimate victory and all of his elect will be saved. None will be lost and mortal man need not be feared as he has no power over our eternal destination.

Hebrews 13:6 So we can confidently say, “The Lord is my helper; I will not fear; what can man do to me?”
John 8:32 and you will know the truth, and the truth will set you free.”

Verse 3. #3: The Prince of Darkness grim, we tremble not for him;

With the full armor of God in place we need not fear the prince of this world.

Ephesians 6:13 Therefore take up the whole armor of God, that you may be able to withstand in the evil day, and having done all, to stand firm.

Verse 3, #4: His rage we can endure, for lo, his doom is sure, One little word shall fell him.

Since we know that Satan is doomed for the lake of fire, we can endure in the sure hope that the victory belongs to the Kingdom of God. Christians can be secure in God’s Word.

Rev. 20:10 and the devil who had deceived them was thrown into the lake of fire and sulfur where the beast and the false prophet were, and they will be tormented day and night forever and ever.