The world in which we live did not develop by chance, nor are humans the result of any form of evolution. There is a Supreme Being which created the universe and subsequently the life forms found upon it. This Creator has no peers or equals. There is no such thing as a pantheon of gods, where the Creator is just one of many. There are not three Gods- only one. This Supreme Being is known in Jewish scriptures as Yahweh, in Christian scriptures as God, and in the Quran as Allah. In other words, Jews, Christians and Muslims believe in and worship the same Supreme Being but they call Him by different names.
Allah created a beautiful world with lovely creatures upon it. These all give glory to the Creator in very special ways. However, Allah also made a unique species of life form- man and woman. These were given free will to either glorify Allah in worship and praise or to ignore the Supreme Being and live selfish lives. Allah is pleased when we pray to Him- giving thanks for all that He has done and asking Him to help us live for Him. Allah is also pleased when we give up things that are important to us in order to show our love and devotion to Him. This form of worship can be manifested by fasting (not eating food for a certain period of time); giving financially to charities (zakat in Islam, tithing 10% of income in Christianity); sacrificing an animal and giving food to the poor; and through the effort of making a pilgrimage to holy places. In short, Allah is pleased when we live our lives for Him.
Human beings, by their nature, do not live forever on this earth. One hundred years from now every person who is reading this sentence right now (including me!) will have experienced physical death. Billions of people now alive: the rich businessman, the famous athlete, the popular entertainer, the drunkard husband, the unfaithful wife- all of these and many more types of individuals will have died. Although memories of them- either good or bad- will fade over time, how they acted towards Allah will have a permanent affect on their eternity as spirit-types of beings.
When a man or woman dies their souls live on. They will eventually go to a final resting place that is commonly referred to in the monotheistic religions of Judaism, Christianity and Islam as heaven and hell. Whether heaven is viewed as a large city with streets of gold or a lush garden with abundant pleasures, what makes heaven a paradise is that it is the place where Allah will be present. On the other hand, the burning flames, pain and endless misery associated with those souls in hell are simply manifestations of a place where Allah's presence will not exist.
The question that has been discussed and debated for thousands of years is how can an individual be sure that s/he will go to heaven after s/he dies? It is not by calling yourself a member of a certain religion. It is not by saying the right kinds of things. It is not by simply being a nice person. Heaven is a place where Allah can be worshiped by those individuals who worshiped Him on earth. In other words, our current life is simply a test from Allah to determine who will spend eternity with Him and other believers in paradise. Those men and women who care nothing about the Supreme Being; who do not want to give praise and glory to the One who created them; who do not want to live holy lives that please Allah; who desire to live only for themselves by indulging in sinful and self-fulfilling pleasures, these multitudes will spend eternity in hell. By not caring about Allah on this earth, they will be sentenced to spend the rest of time in a place where Allah's presence will be missing.
A man who is drunk can fast, a married woman leaving her boyfriend's bed can pray, and a youth who steals money from a friend can give it to charity, but none of their outward acts of worship will be accepted by Allah. All of us living on this earth have done things which were wrong: cheating, lying, gossiping, being selfish, hating others and wanting to take revenge against them. Although they can be called "mistakes" the Supreme Being views them as "sins." Sins are simply things we say and do which prevent us from worshiping Allah the way He desires to be praised. Allah is a Holy God who does not want to have sin in His midst. Sin separates us from Allah and acting sinful will result in eternal separation from Him in hell. Just as giving someone a piece of bread is only proper with clean- not dirty- hands, so Allah is most pleased when we praise Him with hearts and hands not stained by sin and wrong behavior.
In the major monotheistic religions of Judaism, Christianity and Islam, the notion of Yahweh/God/Allah being pleased with a form of blood sacrifice is common. The practice of this sacred act involved a perfect, unblemished, sinless sacrifice to have their blood flow to atone for (take the place of) those who are imperfect, blemished and sinful. So the symbolism is for the pure to become a substitute for the impure so that they may then subsequently be viewed as pure themselves. I am not saying that the Supreme Being could not forgive sins in a different way, just that this ritual has been a common thread throughout the Holy Books. Originally, animal sacrifices were required but they were insufficient in the sense that they did not have knowledge of what was actually taking place and the giving of their lives was not voluntary. Subsequently, the Supreme Being decided to send a Messiah to mankind that would be perfect, unblemished and sinless. It was the task of this Messiah to remain sinless until the time he would freely choose to make his life a sacrifice for others: the perfect for the imperfect. Because of the special circumstances surrounding this event, there would only need to be one such act of atonement for human beings throughout the ages to choose to either accept or reject.
One of the ways an animal sacrifice was symbolically shown to be accepted by the individual it was benefiting was by placing a small drop of blood on that person's forehead. This was a sign of the blood's substitution being accepted. In a similar way, it is necessary for a person to pray to Allah and acknowledge their acceptance of the Messiah's sacrifice for sin as it will apply to their lives. They are to ask Allah to forgive their past sins and wrongdoings. They must also be willing to repent of their sin, meaning that they will strive not to repeat these actions and, instead, act the way Allah wants them to behave. Just as evil spirits can influence a person from the inside-out so also can a holy spirit be allowed to have control of a believer's life. This means that after a person receives forgiveness from their sin they should ask Allah to fill them with His Spirit to help them do right and not wrong. So by prayer the believer asks Allah to cleanse out the bad in their life and then to fill up those now empty spaces with His spiritual presence. This process can be compared to the owner of an apartment who first evicts the bad tenant who is doing harm to their property (i.e., repenting of sin), then cleans up the apartment of all the dirt and filthy walls that were left (i.e., asking for forgiveness), and then rents out the clean and newly painted flat to tenants who will take good care of his home (Allah's Holy Spirit now in control of the believer's life).
The main reason why it is important to experience forgiveness of sin and invite the presence of Allah in your life is that it subsequently allows for proper worship of the Supreme Being. There will be no sin in heaven- Allah will not allow it. Therefore, He requires that those who will worship Him in heaven (by worshiping Him first on earth) be freed from the control of sin. This is not saying that they will never do wrong again, rather, that with the presence and assistance of Allah's Spirit they can live a life closer to what the Supreme Being desires from His human creations. In other words, they can become better submitters to Allah's will for their lives (what being a Muslim actually means). Isa, known also as Jesus, is the Messiah. By combining the sacrifice of sin provided by Isa with the general tenets of submission to the will of Allah (Islam), a Muslim can have a greater degree of assurance that the burden of sin, which hinders the worship of Allah, can be done away with allowing for Allah's Spirit to create a truer and purer form of worshiping the Creator.