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.
Luke 17:20-30
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Being asked by the Pharisees when the Kingdom of God would come, he answered them, "The Kingdom of God doesn't come with observation;
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neither will they say, 'Look, here!' or, 'Look, there!' for behold, the Kingdom of God is within you."
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He said to the disciples, "The days will come, when you will desire to see one of the days of the Son of Man, and you will not see it.
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They will tell you, 'Look, here!' or 'Look, there!' Don't go away, nor follow after them,
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for as the lightning, when it flashes out of the one part under the sky, shines to the other part under the sky; so will the Son of Man be in his day.
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But first, he must suffer many things and be rejected by this generation.
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As it happened in the days of Noah, even so will it be also in the days of the Son of Man.
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They ate, they drank, they married, they were given in marriage, until the day that Noah entered into the ship, and the flood came, and destroyed them all.
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Likewise, even as it happened in the days of Lot: they ate, they drank, they bought, they sold, they planted, they built;
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but in the day that Lot went out from Sodom, it rained fire and sulfur from the sky, and destroyed them all.
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It will be the same way in the day that the Son of Man is revealed.
