Whoever is killed for the sake of Allaah is a martyr. What is meant by “for the sake of Allaah” is that he is fighting so that the word of Allaah will be uppermost. This is the definition of the martyr killed in battle, as stated by the Prophet (peace and blessings of Allaah be upon him).
Al-Bukhaari (3126) and Muslim (1904) narrated that Abu Moosa al-Ash’ari (may Allaah be pleased with him) said: A Bedouin said to the Prophet (peace and blessings of Allaah be upon him), “A man fights for the sake of booty, and a man fights so that he will become famous, and a man fights to show off – who is fighting for the sake of Allaah?” He said, “The one who fights so that the word of Allaah will be uppermost is the one who is fighting for the sake of Allaah.”
Muslim (1915) narrated that Abu Hurayrah said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Who do you think is a martyr among you?” They said, “O Messenger of Allaah, the one who is killed for the sake of Allaah is a martyr,” He said, “The martyrs of my ummah would be few in that case.” They said, “Then who are they, O Messenger of Allaah?” He said: “The one who is killed for the sake of Allaah is a martyr; the one who dies for the sake of Allaah is a martyr; the one who dies of the plague is a martyr; the one who dies of a stomach disease is a martyr; and the one who drowns is a martyr.”
So the Prophet (peace and blessings of Allaah be upon him) mentioned both the martyrs who die in battle and those who also share the virtue of martyrdom.
Al-Nawawi (may Allaah have mercy on him) said: The scholars said: What is meant by the martyrdom of these people who are not killed for the sake of Allaah is that in the Hereafter they will have the reward of martyrdom, but in this world their bodies should be washed and the funeral prayer should be offered for them. We have already discussed this in Kitaab al-Eemaan. The martyrs are of three types: the martyr in this world and the Hereafter, who is the one who is killed in war against the kuffaar; the martyr in the Hereafter, who is not treated as a martyr as far as rulings in this world are concerned, who are the people mentioned here; and the martyr in this world who is not regarded as a martyr in the Hereafter, which is the person who steals from the war booty or who is killed when running away from the battlefield.
Martyrdom is a high status, which can only be attained by the believers (Muslims).
With regard to the one who heard the call of the Prophet Muhammad (peace and blessings of Allaah be upon him) and did not believe in him, he is a kaafir who disbelieves in Allaah and is one of the people of Hell, whether he was killed in battle or died in his bed, whether he was a Jew or a Christian or any other religion of shirk and kufr. The Christians are kaafirs because they reject the Prophet (peace and blessings of Allaah be upon him) and because they claim that the Messiah ‘Eesa ibn Maryam (Jesus the son of Mary) is divine, and because they claim that God has taken a wife and a son – exalted be Allaah far above what they say.
Allaah says (interpretation of the meaning):
“Verily, those who disbelieve in Allaah and His Messengers and wish to make distinction between Allaah and His Messengers (by believing in Allaah and disbelieving in His Messengers) saying, “We believe in some but reject others,” and wish to adopt a way in between.
They are in truth disbelievers. And We have prepared for the disbelievers a humiliating torment”
“And the Jews say: ‘Uzair (Ezra) is the son of Allaah, and the Christians say: Messiah is the son of Allaah. That is their saying with their mouths, resembling the saying of those who disbelieved aforetime. Allaah’s Curse be on them, how they are deluded away from the truth!”
“Surely, they have disbelieved who say: ‘Allaah is the Messiah [‘Eesa (Jesus)], son of Maryam (Mary).’ But the Messiah [‘Eesa (Jesus)] said: ‘O Children of Israel! Worship Allaah, my Lord and your Lord.’ Verily, whosoever sets up partners (in worship) with Allaah, then Allaah has forbidden Paradise to him, and the Fire will be his abode. And for the Zaalimoon (polytheists and wrongdoers) there are no helpers”
“Surely, disbelievers are those who said: ‘Allaah is the third of the three (in a Trinity).’ But there is no Ilaah (god) (none who has the right to be worshipped) but One Ilaah (God —Allaah). And if they cease not from what they say, verily, a painful torment will befall on the disbelievers among them”
And the Prophet (peace and blessings of Allaah be upon him) said: “By the One in Whose hand is the soul of Muhammad, no one of this nation, Jew or Christian, hears of me then dies without having believed in that with which I was sent, but he will be one of the people of Hell.” Narrated by Muslim, 153.
The above comments have to do with the rulings on his situation in the Hereafter if he is killed. With regard to the rulings on his situation in this world, if a non-Muslim joins the fight to defend a Muslim country without that involving any compromise on religious matters on the Muslims’ part, then there is nothing wrong with thanking him and rewarding him for that.
And Allaah knows best.