I agree, Chette, it is pretty clear these martyrs were believers. If someone compares the order of the seven seals in Revelation with the order of events in Matthew, you will see that those souls under the altar were the Tribulation martyrs killed by the Antichrist and his followers.

Matthew 24:9-10 Then shall they deliver you up to be afflicted, and shall kill you: and ye shall be hated of all nations for my name's sake. And then shall many be offended, and shall betray one another, and shall hate one another.

Revelation 6:9-11 And when he had opened the fifth seal, I saw under the altar the souls of them that were slain for the word of God, and for the testimony which they held: And they cried with a loud voice, saying, How long, O Lord, holy and true, dost thou not judge and avenge our blood on them that dwell on the earth? And white robes were given unto every one of them; and it was said unto them, that they should rest yet for a little season, until their fellowservants also and their brethren, that should be killed as they were, should be fulfilled.