325. I saw under the altar the souls of those who had been slain for the word of God and for the testimony which they held. This symbolizes people who were hated by the evil, treated with scorn and expelled because of their life in accordance with the Word's truths and their acknowledgment of the Lord's Divine humanity, and who were protected by the Lord to keep them from being led astray.
"Under the altar" symbolizes a lower earth where the inhabitants were protected by the Lord. An altar symbolizes worship of the Lord out of the goodness of love.
The souls of those who had been slain mean here, symbolically, not martyrs, but people who were hated, treated with scorn, and expelled by the evil in the world of spirits, and who could be led astray by followers of the dragon and by heretics.
"For the word of God and for the testimony which they held" means, symbolically, "because of their life in accordance with the Word's truths and their acknowledgment of the Lord's Divine humanity." Testimony in heaven is given only to people who acknowledge the Lord's Divine humanity, for it is the Lord who testifies, and who enables angels to testify (no. 16); "for the testimony of Jesus is the spirit of prophecy" (Revelation 19:10).
 Since the souls were under the altar, it is apparent that they were being protected by the Lord. For the Lord protects all people who have lived some life of charity, to keep them from being harmed by the evil; and after the Last Judgment, when the evil have been removed, they are released from their asylums and elevated into heaven. I have often seen them after the Last Judgment being let out of the lower earth and conveyed into heaven.
 The fact that those who are slain mean people who are expelled, treated with scorn, and hated by the evil in the world of spirits, and who can been led astray, as also people who wish to know truths, but cannot because of the falsities in the church - this can be seen from the following passages:
Thus said the Lord... God, "Feed the sheep for slaughter, whose owners slaughter them... So I fed the sheep for slaughter because of you, you poor of the flock." (Zechariah 11:4-5, 7)
...we are slain all day long; we are accounted as a flock for the slaughter... Do not forsake us, O Jehovah! (Psalms 44:22-23)
Those who are coming, Jacob will cause to take root... Has He been slain according to the slaughter of his slain? (Isaiah 27:6-7)
...I have heard... the voice of the daughter of Zion..., saying, 'Woe is me..., my soul is weary because of the murderers!" (Jeremiah 4:31)
...they will deliver you up to tribulation and kill you, and you will be hated... for My name's sake. (Matthew 24:9, cf. John 16:2-3)
The Lord said this last to His disciples, but by disciples He meant all who worship the Lord and live according to His Word's truths.
 The evil in the world of spirits continually wish to kill these people. But because they cannot do so physically there, they continually try to do so as regards the soul. And when they cannot do this, they burn with such hatred against these people that they feel nothing more delightful than to do them harm. The reason [they cannot kill them] is that the Lord protects them, and when the evil are cast out into hell, which happens after the Last Judgment, they are brought out of their asylums. But see the explanations to chapter 20, and no. 846 there regarding these people.
That killing or slaying in the Word symbolizes the destruction of souls, which is to kill spiritually, is apparent from many passages there, including also the following: Isaiah 14:19-21; 26:21; Jeremiah 25:33; Lamentations 2:21; Ezekiel 9:1, 6; Revelation 18:24.
(References: Revelation 6:9-11)