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.

1893. That 'Sarai, Abram's wife, bore him no child' means that the Rational Man did not as yet exist will be clear from what is said later on, when Isaac is the subject, for everyone, as has been stated, has an internal man, a rational man which is in between, and an external man, which strictly speaking is the natural man. These, as they existed with the Lord, were represented by Abraham, Isaac, and Jacob - the Internal Man by Abraham, the Rational Man by Isaac, and the Natural Man by Jacob. The Lord's Internal Man was Jehovah Himself, for He was conceived from Jehovah. This was why so many times He referred to Jehovah as His Father, and why in the Word the Lord is called 'the only begotten of God' and 'God's only Son'. The rational man does not exist with anyone when he is first born, only a potentiality to become rational, as may become clear to anyone from the fact that new-born babes do not possess reason but become rational as time goes by through the response of the senses to stimuli from without and from within, as knowledge and cognitions are bestowed on them. Rationality does, it is true, appear to exist with children; but rationality does not in fact do so, only something of the first beginnings of it, as may be recognized from the fact that reason resides with people who are adult and advanced in years.

[2] The Lord's Rational Man is the subject in the present chapter. The Divine Rational itself is represented by Isaac, but the first rational before it had become Divine is represented by Ishmael. Here therefore the statement that 'Sarai, Abram's wife, bore him no child' means that the Divine Rational did not as yet exist. As stated already, the Lord was born in the same way as any other, and as regards what He derived from Mary His mother He was like any other. And because the rational is formed through facts and cognitions which enter in by way of the external senses, or the senses that belong to the external man, His first rational was therefore born as it is with any other. But since everything human in Him was made Divine by His own power, so was the rational made Divine. His first rational is described in the present chapter, and once more in Chapter 21, where again in verses 9-21 Hagar and Ishmael are the subject, where it is said that Ishmael was cast out when Isaac, who represents the Divine Rational, had grown up.