Atonement for sins is a foundational concept in the comparison of the three "great faiths". The principle comes from the Torah of Judaism which is referred to as the Old Testament in Christianity.
In the Torah/Old Testament book of Numbers chapter 5, we are told that sin is being unfaithful to the Lord. Throughout the Torah, we find that there are two types of sins.
There are sins of commission which are basically things that people do even though God says to not do them. Then, there are sins of omission which are things that people do not do which God has said to do.
These sins are defined by laws that God has given to men. Christians refer to the law as the Ten Commandments while Orthodox Judaism consists of 613 laws. Islam has laws for every aspect of a Muslim's life as well as the government and community.
In Leviticus chapter 16, God gave the Israelites the procedure for the atonement for sins of the people. The ceremony was performed by the high priest. This involved a process of animal sacrifice where the blood of the animals was substituted for the blood of the sinner. This blood was sprinkled on the kapporeth "mercy seat" which was the cover for the ark of the covenant. This cover was made of solid gold that was hammered into shape. This ark was a box made of acacia wood which, in Israel, is known to be the incorruptible wood. The box was overlaid with gold which represents divinity or deity throughout the Bible. Inside the box were the stone tablets that the Ten Commandments were written on, Aaron's rod that budded, and a golden pot of manna ("angels food"). God's people were not allowed to open the cover and look inside.
Then, the sins of the people were confessed and placed on the head of a goat. That goat was referred to as a scapegoat. The goat carried away the sins of the people. It was sent out of the camp and released into the desert.
This Day of Atonement was observed once every year. At other times throughout the year, individual sacrifices were made for each person as necessary for their sins. If someone died with sins that were not atoned for they would be eternally separated from God
In Orthodox Judaism, the time of atonement is called The Days Of Awe. It starts on Rosh Hashanah which is the first day of the year and continues for ten days. The tenth day is called Yom Kippur which means day of atonement.
It is believed that, on Rosh Hashanah, God writes the names of those who will live, die, have a good year, and have a bad year in books. It is believed that, during the Days Of Awe, it is possible to alter God's decrees through repentance, prayer, and charity. On Yom Kippur the books are sealed for the coming year. This process is only used for sins committed between man and God. For sins committed between men, the people must seek reconciliation and right the wrongs.
Chasidic Jews observe a process of atonement for sins called Kapparot. This is where a live fowl is purchased and waved over the head while reciting a prayer for the sins. The fowl is then slaughtered and it (or its equivalent value) is given to the poor.
In Orthodox Judaism, the animal sacrifices were ended in the year 70 C.E. This was the time where the Roman army destroyed the Temple. The rituals were stopped due to the belief that it would be a sin to perform the sacrifices anywhere besides the Temple.
It is believed that there are some Orthodox rabbis in Israel who practice the ritual sacrifices. They do it so that the knowledge will not be lost. According to Judaism, when the Messiah comes a new place will be provided to perform the sacrifices.
The Days Of Awe is an annual event for the atonement for sins. It is performed by all the Jewish people. There is no assurance of which book your name is written. Therefore, you are never assured of a right relationship with God.
In the Qu'ran, atonement for sins is made through the actions of the people. In surah 005-045, atonement is made by forgiving others. In surah 005-095, expiation (atonement) is granted by feeding the poor as well as fasting.
The month of Ramadan is a special time in Islam. It is a time of generosity and sacrifice that is observed every year. It is believed that the good deeds are sent ahead of them into heaven to replace the sins. The month of Ramadan is observed by all Muslims each year.
A Muslim is not sure of their relationship with God because they do not know if they have enough good works to make up (atone) for their sins.
In Christianity, atonement for sins is a gift from God. Unlike the annual actions, the gift is given once for all and there is no need for repeating. The ark of the covenant is a picture of Christ and His work on the cross. The fact that the ark was made of wood covered with gold speaks of the fact that Jesus was fully human and at the same time fully God. The things inside represent man's rebellion against God's laws, His appointed leaders, and His provision which is the complete manifestation of our sins. The cover is a picture of the fact that only the perfect sacrifice of God's own Son could permanently remove our sins. Just as it was hammered into shape to cover the ark, Jesus was beaten (hammered) and suffered for our sins. He, in fact, became the mercy seat and His blood covered all.
In The New Testament book of Hebrews, chapter nine, we are told that Christ came as the high priest. We are also told that through His perfect sacrifice we have been made holy once and for all.
Jesus Christ fulfilled the requirements of the Torah therefore no annual ceremony is required. What is required is that His gift must be accepted.
Christians are assured of the atonement for sins and that they have a right relationship with God. We are even given another gift, called the Holy Spirit, to assure us of that fact.