Altar
The first altar mentioned in the Word was the one built by Noah after he came out of the ark, after being saved from the great flood. On that altar, he sacrificed clean animals to the Lord.
Mountains represent the Lord because of their height; we need to raise our thoughts above worldly things when "talking" with the Lord. An altar is a small artificial mountain. When it's used in worship, it can call to mind this raising of thought. The fire and smoke that rise from an altar are symbolically being sent to the Lord.
Most altars were made from unhewn stones. Stones represent truths. Unhewn stones - ones that have not been shaped by men - represent truths from the Word, truths that have not been adulterated.
The clean beasts to be sacrificed represent good things, charitable acts done because they are right. The clean birds represent thoughts about doctrine and actions, and about what is right. Presenting these things is an acknowledgment that we have them from the Lord, and a giving thanks to Him for them.
In the Israelitish Tabernacle, the altar of burnt offering represented the acknowledgment of good and the altar of incense that of truth. For this reason this larger altar, which was outside by the door, was made of brass which signifies natural good, while the altar of incense was made of gold, which signifies love to the Lord from whom comes truth.
Numbers 15:2-15
2
"Speak to the children of Israel, and tell them, 'When you have come into the land of your habitations, which I give to you,
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and will make an offering by fire to Yahweh, a burnt offering, or a sacrifice, to accomplish a vow, or as a freewill offering, or in your set feasts, to make a pleasant aroma to Yahweh, of the herd, or of the flock;
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then he who offers his offering shall offer to Yahweh a meal offering of a tenth part [of an ephah] of fine flour mixed with the fourth part of a hin of oil:
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and wine for the drink offering, the fourth part of a hin, you shall prepare with the burnt offering, or for the sacrifice, for each lamb.
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"'Or for a ram, you shall prepare for a meal offering two tenth parts [of an ephah] of fine flour mixed with the third part of a hin of oil:
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and for the drink offering you shall offer the third part of a hin of wine, of a pleasant aroma to Yahweh.
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When you prepare a bull for a burnt offering, or for a sacrifice, to accomplish a vow, or for peace offerings to Yahweh;
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then shall he offer with the bull a meal offering of three tenth parts [of an ephah] of fine flour mixed with half a hin of oil:
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and you shall offer for the drink offering half a hin of wine, for an offering made by fire, of a pleasant aroma to Yahweh.
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Thus shall it be done for each bull, or for each ram, or for each of the male lambs, or of the young goats.
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According to the number that you shall prepare, so you shall do to everyone according to their number.
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"'All who are native-born shall do these things in this way, in offering an offering made by fire, of a pleasant aroma to Yahweh.
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If a stranger lives as a foreigner with you, or whoever may be among you throughout your generations, and will offer an offering made by fire, of a pleasant aroma to Yahweh; as you do, so he shall do.
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For the assembly, there shall be one statute for you, and for the stranger who lives as a foreigner [with you], a statute forever throughout your generations: as you are, so shall the foreigner be before Yahweh.
