What is wrong with the Bible that Muslims feel it necessary to have a new scripture?
It is not the case that suddenly someone appeared that said "I'm going to make up a new thing and call myself a Muslim, right let's have a new scripture". More on that later.
The scripture given to Jesus (peace be upon him) and the Torah, are no longer in their original forms. The current day Bible is mainly the words of men, some of whom never met Jesus, some of whom aren't even known. Christian beliefs deviated from what Jesus (peace be on him) preached.
The Qur'an is the last and final scripture of God, the very words revealed by God to Prophet Muhammad (peace be upon him) via the angel Gabriel. It's message is for the entire world, until the world ends. It is because Allah has promised to preserve the Qur'an that there won't be any need for a new Prophet, because the message is, and will remain, intact. Thus it is fully preserved with no word in it coming from any human. It is 100% the word of God, unlike the other scriptures that we have with us today, that have been changed by humans - thus it supersedes the previous scriptures, while being a continuation, confirmation and culmination of the original message contained within them, in its last and final form. The Final Testament. It has no versions or editions. Millions of people from all over the world have it memorised and they all recite word for word the same thing. It contains the central message as already described, and practical guidance on how to live, stories of previous prophets from which to learn lessons, warnings, rules, comfort, solace, good tidings, and in it God corrects misconceptions people may have about Him or His prophets. It tells us what has always been expected from humans since the beginning of time; what He told His prophets to teach people since the beginning. That message never changed. The essence of Islam is what always was, and has always been, the true and natural religion; the way of all the Prophets, the original and only message.
The Qur'an has been translated into nearly every language. These are regarded as translations of the Qur'an, but not as the Qur'an itself, which is in Arabic, and has rhythm and poetic beauty. Any preconceived notion that you might have of a book should be put aside. This book is completely unique in every way; its content, arrangement, style, authorship, preservation, and memorisability, amongst other things. The Prophet's practice and directives (the Sunnah), as taught to him by God, have also been recorded and preserved, and must be followed along with the Qur'an.
I have always been taught that Jesus is the son of God, and with the Holy Spirit God is three persons and one God. I never really understood it but it is scripture.
The reason you haven't understood it, is because it doesn't resonate with the heart, doesn't agree with humans naturally and doesn't make sense.
In Christianity, depending on who you ask, either 3=1 or 1=3. In Islam 1=1. The concepts are pure, clear, agree with the heart and mind and logic, and make sense. The Qur'an repeatedly asks people to ponder and reflect. in Islam:
There is only One God. He alone should be worshipped. He is our Creator, Sustainer, and Lord.
He does not beget, nor is He begotten. He has no sons, daughters, spouses, siblings, parents, cousins, or relatives of any sort.
He is eternal and does not die. He does not depend on anyone/anything yet we all depend on Him. He is free of all want and need.
There is nothing like Him. He is all Hearing, all Seeing, all Knowing, all Powerful, Incomparable, the Creator of the Universe.
He did not/does not, become flesh, dwell in human or animal bodies, nor are there any incarnations of Him. He is not mixed up in His creation in any way.
He is not composed of persons, nor a trinity. There are no secondary, lesser, greater, equal, or multiple gods, nor any intermediaries. And no denying of God's existence either.
There are no sharers, associates, persons or parts whatsoever in His exclusive Divinity. Simply, He is One, in every sense.
As you can see, the concept is clear, simple, logical, makes sense, and befits the Majesty of God. The proper name of God in Arabic is Allah.
How and why would the coming of Muhammad be necessary when the Bible was closed after Jesus came and spoke God's truth and died for our atonement and resurrected?
Allah has sent a succession of prophets to people throughout the ages, to convey His message to them, and with guidance to show people how He wants them to live and worship Him. Muslims believe in all the prophets Allah sent, and do not reject or blaspheme any of them, from Adam, to the last and final prophet, Muhammad, peace be on them all. They were the purest and noblest of humanity and were not divine in any way. Allah sent all the prophets with the same message and not different messages. The message was:
to submit wholeheartedly to Allah and worship Him and Him alone, without any associates in, or parts to, His Exclusive Divinity, and to obey the prophet. They taught that people should be under no misperception that they can commit themselves to Allah as their Lord, and then combine this with accepting others as their Lord, or associating others in His Divinity, in whatever way. They taught that we should strive hard to translate our belief in the One True God into practice, by obeying Allah and the messengers He sent, who were also role models and examples for us, showing us practically how to put the guidance they were sent with into practice in our daily lives, explaining the scriptures, warning against wrong-doing, giving good tidings, and giving additional legislation from Allah.
So Islam is not a new faith, but is
the same ultimate universal truth that God revealed to all the prophets, including Noah, Abraham, Moses and Jesus (peace be on them), and the same thing they all taught. Thus Islam is not named after a specific person (like Christianity, Buddhism), nor after a certain race or place (like Judaism, Hinduism), but is named by God Himself, the meaning loosely translating as 'submission to God', which is what every Prophet and their righteous followers did, from amongst all times, places and peoples. That in itself is one fraction of the evidence that it was the way of all the Prophets from the beginning.
With time, the message got forgotten or corrupted. So people started worshipping other gods along with Him, made idols, said that God begot a son, said that certain people were incarnations of God, some rejected belief in God altogether, while others elevated the status of some prophets to divine, or at the other end, rejected or blasphemed some of the prophets. Whenever God's message got distorted by people, or forgotten, a new messenger was sent, not with any new or changed message, but reinforcing the actual message that God sent all the messengers with, the actual core beliefs that people were taught from the beginning of humanity, confirming the true parts of previous teachings and scriptures, and correcting wrong beliefs and misconceptions that had crept in. God required that whenever He sent a new messenger, that messenger should be followed along with any new scripture given to him. This chain of messengers culminates in prophet Muhammad (peace on him), who is the last and final prophet and messenger. Since his prophethood, God's message is available unchanged and unadulterated, for the entire world, until the end of time. He wasn't sent as prophet and messenger for a specific group of people and specific time (e.g. as Moses and Jesus were to the Children of Israel), but he was sent for all the world, for all time, until the Day of Judgement. Thus he is the last, not first, prophet of Islam; a messenger to all mankind, for now, and for all time to come. He is the messenger who must now be followed.
The message didn't suddenly change when it came to Jesus (peace on him), nor did he rebel against God and claim divinity for himself. In the fifth chapter of the Qur'an there are the following verses, which foretell what Jesus (peace on him) will say when God, on the Day of Judgement, to illustrate to humankind, will ask Jesus whether he claimed divinity or said he should be worshipped:
"He [Jesus] will say, "Exalted are You! It was not for me to say that to which I have no right. If I had said it, You would have known it. You know what is within myself, and I do not know what is within Yourself. Indeed, it is You who is Knower of the unseen. I said not to them except what You commanded me - to worship Allah , my Lord and your Lord. And I was a witness over them as long as I was among them; but when You took me up, You were the Observer over them, and You are, over all things, Witness." (5:116-117)
died for our atonement and resurrected?
We are each responsible for and accountable for our own deeds, good or bad, and for the choices we make in life, each choice having a consequence. Nobody else is responsible for them or carries them away. No innocent person is made to suffer or die for other's sins. Babies are not born in a state of sin, but a state of innocence and purity. There is no concept of original sin, nor of God expecting perfection which cannot be achieved, nor of a broken relationship with God that requires reconciling, nor of ****ation requiring a saviour. Forgiveness is directly from God. Prayer is directly to God. God is the God of all, even of those who don't believe in Him.
In Islam, as Adam (peace be upon him) asked for forgiveness and was forgiven, so we too ask for Allah's forgiveness for our sins, as He loves for us to turn to Him in repentance, and loves forgiving. This forgiveness comes freely, just by Allah's will, when we sincerely ask for forgiveness and truly repent. Forgiveness does not require any type of sacrifice by God. Both Adam and Eve repented and were forgiven by their Merciful Lord; and indeed Adam was then chosen to be the first person to receive guidance from Allah, was honoured by Allah, and is counted among all the other Prophets of Islam. While guidance and the right way has been shown to us, we, as humans, have the freedom to choose, to err, and to repent sincerely, and should we do so, we will find Allah Forgiving. For all in the posterity of Adam, the door of returning to the right path is always open, prior to death.
With regards to salvation and sin, my understanding is that Christianity hinges on the fact, that according to Christians, God did not forgive Adam (peace be upon him) for eating from the tree, and that not only Adam but all subsequent generations have to bear a punishment from God, and fell out of grace with God, their relationship with God was severed, they are born into a state of sin, and death entered the world. They believe that God expects perfection from humans which humans can never achieve (partly because of the aforementioned sin), thus to atone for our imperfection and inability to meet God's expectation, and to reconcile people to God and repair the severed relationship, and to forgive (but in reality atone) people's sins, the sacrifice of an innocent perfect man (who also happens to be God) and his slow, bloody, and painful death, and thereafter resurrection, comes into play.
God forgave Adam (peace be upon him), and guided him, and has sent guidance for us all, and can forgive us all, freely, when we repent sincerely and ask sincerely, resolving not to repeat past mistakes, and does not expect perfection from us, but simply for us to worship Him alone without any associates, persons or parts to His Exclusive Divinity, and to not ascribe divinity to anybody or anything else, and to do our very best and try our very hardest with all our heart, body and soul, to love and obey Him and the messengers He sent. His Mercy opening the doors to salvation and paradise.
It is actually denigrating to God's Power that He should not be able to forgive or remove sins without begetting a son and then sacrificing the son. It ascribes imperfection to God. This is not forgiveness, but atonement. One thing pays the price for another. Perfection is being able to forgive, freely, abundantly, at will. That is perfection. Islam recognises God's power to forgive with just His Will. Some of the other religions seem to claim that forgiveness requires a purchase price and if we can't pay it, somebody else has to pay it on our behalf. In Islam, forgiveness comes without a purchase price or sacrifice. We do not/cannot: buy, sacrifice for, earn or steal it. It comes freely when we sincerely ask for forgiveness and truly repent, accepting God's will.