The phrase "under the altar" appears in the Book of Revelation, specifically in Revelation 6:9-11. This imagery is rich in symbolism and has been the subject of much theological reflection and interpretation within Christian eschatology.

Biblical Reference:

Revelation 6:9-11 states: "When the Lamb opened the fifth seal, I saw under the altar the souls of those who had been slain for the word of God and for the testimony they had upheld. They cried out in a loud voice, 'How long, O Lord, holy and true, until You avenge our blood and judge those who dwell upon the earth?' Then each of them was given a white robe and told to rest a little while longer, until the full number of their fellow servants, their brothers, were killed, just as they had been."

Symbolism and Interpretation:

1. The Altar:
In the context of the Old Testament, the altar was a place of sacrifice and worship. It was central to the sacrificial system established by God for the Israelites. The altar in the heavenly vision of Revelation signifies a place of divine presence and justice. It is where the sacrifices of the faithful are acknowledged and remembered by God.

2. Souls of the Martyrs:
The souls under the altar represent those who have been martyred for their faith in Jesus Christ. Their presence under the altar suggests a close connection to the sacrificial system, as their lives were given in testimony to the Word of God. This imagery underscores the idea that their sacrifice is precious and honored in the sight of God.

3. Cry for Justice:
The martyrs' cry, "How long, O Lord, holy and true, until You avenge our blood," reflects a deep yearning for divine justice. This plea is not one of personal vengeance but a call for God's righteous judgment against evil and the vindication of His faithful servants. It echoes the imprecatory psalms and the prophetic tradition of seeking God's intervention against injustice.

4. White Robes:
The giving of white robes symbolizes purity, victory, and honor. In the apocalyptic literature, white garments often represent the righteousness of the saints and their vindication before God. The robes are a sign of their acceptance and reward in the heavenly realm, affirming their faithfulness and the promise of eternal life.

5. Rest and Fulfillment:
The instruction to "rest a little while longer" indicates that God's plan is unfolding according to His divine timetable. It reassures the martyrs that their sacrifice is not in vain and that ultimate justice will be served. The mention of a "full number" suggests that God has a predetermined plan for the completion of His purposes, including the suffering of His people.

Theological Significance:

The vision of souls under the altar serves as a powerful reminder of the cost of discipleship and the reality of persecution faced by believers. It highlights the tension between the present suffering of the church and the future hope of God's kingdom. This passage encourages believers to remain steadfast in their faith, trusting in God's sovereignty and the promise of His ultimate justice.

The imagery also reinforces the theme of divine retribution and the assurance that God will vindicate His people. It calls the church to a posture of patient endurance, confident in the knowledge that God hears the cries of His people and will act in His perfect timing.