Islam commands this in 4:48 Surah An-Nisa
Indeed, Allah does not forgive associating others with Him in worship,1 but forgives anything else of whoever He wills. And whoever associates others with Allah has indeed committed a grave sin.
Only Allah speaks for Allah, anyone who speaks for him appart from the prophet has commited a great sin, so all Imam who create laws outside of sharia in the name of Allah are not only sinners but grave sinners.
Allah speaks to the hearth of men trough dreams and trought the Quran anything else is man's work.
When Allah created the Quran he was writing it to guide Mohammed and his followers, he never wished for it to be made a organized religion, however Mohammed's family murderers claimed to speak in the name of Allah and created religion and waged war against all even the christians and the Jews this is the Suni and Shia divide the Suni believe it that his son-in-law was to guide the followers and the Shia believed his most Loyal Follower Ali was to guide the people.
To be honest the Quran prohibits murder of muslims, so both were very wrong in fighting between themselves.
They should have followed the Quran and shared in the power under the peace of Allah and the words of the prophet but were tempted to fight by the devil and both claimed to speak for Allah to gain legitimacy commiting a two grave sins for power.
So it depends who you follow:
For Shia Muslim other religions and Suni are Unbelievers
For Suni Muslim other religions and Shia are Unbelievers
On the Quran all who dared speak in the name of Allah are Unbelievers and not to be trusted.
The Quran was writen to guide the Believers, yet after Muhammed all turned against each other turning into unbelievers creating temples instead of praying from their homes to Mecah.
So the true Muslim should hear Allah and Allah only and never claim to speak in his name, or acept others who claim the same to do so is Haram.
He should also read the whole of the Quran and not only the parts he finds to be beneficial to him, so that he won't contradict himself.
So in truth Unbelievers are those who deny the voice of Allah and the Quran is but a guide to help you hear it for yourself trough the experiences of the prophet.
Men will do what they do they will fight and Allah sees no Sin in fighting but he sees sin in killing, specially when it is the killing of those who offer peace.
O believers! When you struggle in the cause of Allah, be sure of who you fight. And do not say to those who offer you ˹greetings of˺ peace, “You are no believer!”—seeking a fleeting worldly gain.1 Instead, Allah has infinite bounties ˹in store˺. You were initially like them then Allah blessed you ˹with Islam˺. So be sure! Indeed, Allah is All-Aware of what you do.
Again Alah will choose those who will receive his words out of the deserving, but is not up for us to choose who believes in it or not only Allah can convince people.
And peraphs Allah has a different path set for them, so we shouldn't question it not knowing were it leads for he is all knowing and merciful.
So an unbeliever just those who refuse the word of Allah, and instead choose to follow the word of Men and be enslaved by it. For many a sin comes from following false prophets. Yet no sin was ever made by a man who trully acepted Allah in his love and mercy as his sheperd and trusted Allah would guide the others on the same path, after all a single sheep can go astray but it will come back in fear but a herd is only lost when sheep start to follow other sheep, so what does Allah say to you with the soul he gave you does it lead to the stony path on the hill or to the grassfields close to the river? Its all a part of his path none are trully lost unless they choose to do so.