Qur'an 4:48— Allah forgiveth not that partners should be set up with Him; but He forgiveth anything else, to whom He pleaseth; to set up partners with Allah is to devise a sin most heinous indeed.
Shirk, according to Islam, is the worst blasphemy, and it is an unforgivable sin. If you swear by something other than Allah, you are guilty of committing shirk.
Sahih al-Bukhari 6108— Allah's Messenger said, "Verily! Allah forbids you to swear by your fathers. If one has to take an oath, he should swear by Allah or otherwise keep quiet."
Sahih Muslim 4040— Allah's Messenger said: "He who has to take an oath, he must not take and oath but by Allah."
Sunan Abu Dawud 3242— The Apostle of Allah said: "Do not swear by your fathers, or by your mothers, or by rivals to Allah; and swear by Allah only, and swear by Allah only when you are speaking the truth."
Sunan an-Nasai 3795— The Messenger of Allah said: "Whoever swears, let him not swear by anything other than Allah."
Sunan Ibn Majah 2095— The Messenger of Allah said: "Do not take oaths by idols nor by your forefathers."
Jami at-Tirmidhi 1534— The Messenger of Allah came across Umar while he was on his mount, and he was swearing by his father. So the Messenger of Allah said: "Verily Allah prohibits you from swearing by your fathers. So let the one who swears, swear by Allah, or be silent."
Why is it wrong to swear by anything other than Allah?
Jami at-Tirmidhi 1535— Ibn Umar said: "Nothing is sworn by other than Allah, for I heard the Messenger of Allah say: 'Whoever swears by other than Allah, he has committed disbelief or Shirk.'"
Whoever swears by anything other than Allah is committing disbelief or shirk. According to Muhammad, anyone who swears by anything other than Allah has made it a partner with Allah. That person has committed shirk, the worst possible sin. Let us look at some of the heinous sins committed by Allah:
Qur'an 38:1— I swear by the Qur'an, full of admonition.
Qur'an 43:2— I swear by the Book that makes clear.
Qur'an 50:1— I swear by the glorious Qur'an.
Qur'an 15:72— Verily, by thy life (O Prophet), in their wild intoxication, they wander in distraction, to and fro. [Allah swears by Muhammad's life.]
Qur'an 77:5— I swear by the angels who bring down the revelation.
Qur'an 79:1— I swear by the angels who violently pull out the souls of the wicked.
Qur'an 81:15— I swear by the stars.
Qur'an 85:1— I swear by the mansions of the stars.
Qur'an 56:75— I swear by the falling of the stars.
Qur'an 91:1— I swear by the sun and its brilliance.
Qur'an 91:6— I swear by the earth and its expanse.
Qur'an 74:32— I swear by the moon.
Qur'an 89:1— I swear by the daybreak.
Qur'an 84:16— I swear by the sunset redness.
Qur'an 92:1— I swear by the night when it draws a veil.
Qur'an 86:11— I swear by the rain-giving heaven.
Qur'an 51:1— I swear by the wind that scatters far and wide.
Qur'an 52:1— I swear by the mountain.
Qur'an 90:1— I swear by this city. [Mecca]
Qur'an 68:1— I swear by the pen.
Qur'an 89:3— I swear by the even and the odd.
Qur'an 95:1-3— I swear by the fig, and the olive, and mount Sinai, and this city made secure.
Qur'an 69:38-39— I swear by that which you see, and that which you do not see. [i.e., everything]
When Allah swore on these things from all eternity in his eternal partner, the Qur'an, he associated all of them with himself, committing more shirk than the worst Meccan polytheist could ever dream of.
Qur'an 2:22— Do not set up rivals to Allah while you know.
Pagans who committed shirk could be forgiven because they did not know any better. The more knowledge you have, the more responsible you are. Allah's knowledge is supposed to be omniscient—all-knowing. In other words, Allah knew exactly what he was doing when he set up literally everything as his partners. Unlike pagans, Allah cannot appeal to ignorance. So, according to Muhammad and the Qur'an, Allah is the worst sinner and the most wicked idolator. And this is who Muslims want us to worship? No thank you! I would rather worship the true and living God of the Bible, Who swears by Himself.
Then the angel of the LORD called to Abraham a second time from heaven, and said, "By Myself I have sworn, declares the LORD, because you have done this thing and have not withheld your son, your only son, indeed I will greatly bless you, and I will greatly multiply your seed as the stars of the heavens and as the sand which is on the seashore; and your seed shall possess the gate of their enemies. Genesis 22:15-17Notice here the difference between the true God, the God of the Bible, and the false god of the Qur'an:
I have sworn by Myself, The word has gone forth from My mouth in righteousness And will not turn back, That to Me every knee will bow, every tongue will swear allegiance. Isaiah 45:23
The LORD has sworn by His right hand and by His strong arm, "I will never again give your grain as food for your enemies; Nor will foreigners drink your new wine for which you have labored." Isaiah 62:8
For if you men will indeed perform this thing, then kings will enter the gates of this house, sitting in David's place on his throne, riding in chariots and on horses, even the king himself and his servants and his people. But if you will not obey these words, I swear by Myself," declares the LORD, "that this house will become a desolation." Jeremiah 22:4-5
Then all the men who were aware that their wives were burning sacrifices to other gods, along with all the women who were standing by, as a large assembly, including all the people who were living in Pathros in the land of Egypt, responded to Jeremiah, saying, "As for the message that you have spoken to us in the name of the LORD, we are not going to listen to you! But rather we will certainly carry out every word that has proceeded from our mouths, by burning sacrifices to the queen of heaven and pouring out drink offerings to her, just as we ourselves, our forefathers, our kings and our princes did in the cities of Judah and in the streets of Jerusalem; for then we had plenty of food and were well off and saw no misfortune. But since we stopped burning sacrifices to the queen of heaven and pouring out drink offerings to her, we have lacked everything and have met our end by the sword and by famine. . . . thus says the LORD of hosts, the God of Israel, as follows: 'As for you and your wives, you have spoken with your mouths and fulfilled it with your hands, saying, "We will certainly perform our vows that we have vowed, to burn sacrifices to the queen of heaven and pour out drink offerings to her." Go ahead and confirm your vows, and certainly perform your vows!' Nevertheless hear the word of the LORD, all Judah who are living in the land of Egypt, 'Behold, I have sworn by My great name,' says the LORD, 'never shall My name be invoked again by the mouth of any man of Judah in all the land of Egypt, saying, "As the Lord GOD lives." 'Behold, I am watching over them for harm and not for good, and all the men of Judah who are in the land of Egypt will meet their end by the sword and by famine until they are completely gone.'" Jeremiah 44:15-18, 25-27
For thus says the LORD, "Behold, those who were not sentenced to drink the cup will certainly drink it, and are you the one who will be completely acquitted? You will not be acquitted, but you will certainly drink it. For I have sworn by Myself," declares the LORD, "that Bozrah will become an object of horror, a reproach, a ruin and a curse; and all its cities will become perpetual ruins." Jeremiah 49:12-13
For when God made the promise to Abraham, since He could swear by no one greater, He swore by Himself, saying, "I WILL SURELY BLESS YOU AND I WILL SURELY MULTIPLY YOU." Hebrews 6:13-14