The Genesis account of the Garden of Eden, and the activity of Cain and Able portray an interesting comparison to the divisions of the Tabernacle in the wilderness and the Temple of of God at Jerusalem. For example, the Most Holy Place is equivalent to the place in the Garden where Adam and Eve met with the Lord (cf. Genesis 3:8), while the rest of the Garden, where Adam and Eve interacted with one another and satisfied themselves with the fruits thereof, would be equivalent to the Holy Place within the Tabernacle or the Temple. Outside the Garden was the Land of Eden, for the Garden was planted within Eden (Genesis 2:8).
When Cain and Able sacrificed before the Lord, they built an altar near the entrance to the Garden, where the Lord was perceived to reside (Genesis 4:3-5; cf. verse 14). This altar would foreshadow the Altar of Burnt Offering, which stood just outside and east of the Tabernacle or the Temple. Cain was upset over the fact the Lord didn’t accept his offering, so the Lord told him that he would be accepted, if he did well, but if not a sin offering (Young’s Literal Translation) lay at the door or gate (Genesis 4:7). The problem is, the word, door (H6607) is ambiguous. Could it mean the door or gate to the Garden, or does it refer to the door or gate to Eden? An addition problem comes into view when we understand that , according to the Law of God, there was no offering for murder or other capital crimes. Men who committed such sins were executed outside the camp. Their own blood was shed as payment for the crime they committed. The Altar of Burnt offering was for lesser crimes that could be forgiven.
When Cain became angry with his brother and killed him, the Lord banished him from Eden. He was driven away from the presence of the Lord and eastward into the land of Nod (H5113), which means wandering. In other words, Cain was driven afar off from the Lord (cf. Ephesians 2:17), which is represented in the gentile nations. Therefore, the Land of Eden is represented by those who are near to the presence of the Lord, like the camp of Israel that settled around the Tabernacle in the wilderness, and there was a gate or door into the camp (Exodus 32:6-7). So, if the Lord pointed to a sin offering at the gate, and this represented the murder in Cain’s heart, he couldn’t have been referring to the gate of the Garden, near to which was the altar Cain and Able were already using. He had to refer to an altar outside Eden, at its entrance.
This altar was typified by the place where the red heifer was led and sacrificed, whose ashes were kept in a clean place outside the camp and mixed with the water of separation, which was used for purifying both the Tabernacle / Temple, the people and anyone who was ceremonially unclean and had to wait outside the camp for a period of time before entering (cf. Numbers 19:1-9). Ezekiel also refers to this place outside the camp in Ezekiel 43:21. Most translators, however, seem to imply this was the alter just outside the Tabernacle / Temple, but this is impossible. Notice that the Altar of Burnt Offering was measured and in the process of being purified (Ezekiel 43:13-20). A bull was taken to a designated place (not the Altar of Burnt Offering) outside the Sanctuary (Ezekiel 43:21). The purifying process took seven days (Ezekiel 43:22-26), and only after that period of time were the priests permitted to offer sacrifices upon the Altar of Burnt Offering. Therefore, the altar where the bull was sacrificed and whose blood was used to purify the Altar of Burnt Offering had to be located elsewhere, outside the Sanctuary.
The book of Hebrews also mentions an altar, which was located outside the camp (Hebrews 13:10-13). It is equivalent to that same altar that all the carcasses of the sin offerings were taken and burnt (Hebrews 13:11), including the red heifer (cf. Numbers 19:1-9). This was the place where Jesus suffered (Hebrews 13:12), and that place was located outside the East Gate of the Temple to a point just outside the city limits on Mount Olivet. That altar had to be located near Golgotha where the men of war came to be counted and purified before entering the city limits. The same is the place where Jesus was crucified (Luke 23:33).