Why does God instruct Noah in every aspect of his salvation—even the exit from the ark (Gen 8:15-19)?

It is true: from Gen 6:13, upon commissioning the ark, until 8:19, when the last animal exits the ark, Noah’s actions are mostly in direct obedience to God’s explicit instructions. There are few times when any individual is so specifically directed by God for such a long period, and for such a momentous purpose. But this certainly makes sense, considering that the salvation of Noah meant the salvation of the human race. Thus God, for something so important, left nothing to chance, i.e., the chancy, fallible powers of human judgment. It is no accident that God chose Noah, a man who walked with God, because Noah was capable of being strictly obedient to God; his fate and ours depended on that holy talent. But more broadly, this divine hand-holding is a precursor and symbol of the precise steps required by redemption through sacrifice, of certain actions taken by prophets (one thinks of how extensively Moses was directed, and of Ezekiel’s exhibitions of prophecy), and later of obedience to the Holy Spirit that every Christian hopes to practice.

Is this the first altar made (Gen 8:20)? What is the function of an altar, anyway?

It does appear to be the first altar in the Bible. If Abel used an altar, we are not told so; if any of the other antediluvian patriarchs used an altar, again we are not told so. The Hebrew is מִזְבֵּחַ, mizbeach, literally, “place of slaughter or sacrifice.” Beyond that we are not really told much. It could have been a heap of stones, logs, or earth. It was probably nothing so elaborate as the altar of the Mosaic code, which was a very specific, and rather different, type of altar, albeit serving a broadly similar purpose.

In Gen 8:20-21, we have references to four features of later sacrifices. What are they and what significance might they have?

Indeed, four features of Mosaic-style sacrifices can be found here, which might suggest anachronism (to a semi-attentive reader), but not necessarily: the author could just as easily be offering the authority of Noah in establishing the legitimacy of certain basic features of Mosaic-style sacrifice, claiming that these were merely the divinely-ordained antecedents of practices later codified. The four features are: an altar (albeit not quite of the sort God tells Moses to make); the use of “clean beasts” and fowl, only, for sacrifice; the use of animals of various kinds for an important sacrifice (here, one of each is offered); and the sacrifices were a “sweet savour” to the Lord, a phrase frequently repeated in Leviticus in describing what sorts of sacrifices God wanted. As to the significance of these practice, first, the altar sets the victim apart, making quite clear what is in the offering. The clean beasts are mostly livestock and poultry, i.e., animals man raises—although, interestingly, some clean beasts, such as deer, though good for food, are not specified as sacrificial victims in the Mosaic code. Clean beast requirement means the animals are among those God is to give the Hebrews for food, hence they are sacrifices indeed, the giving up of food. Using multiple kinds of clean beasts, when only seven were left in the world (plus whatever young were born on the ark), indicates thankfulness for all the food animals that were saved and kept available. As to the “sweet savour,” see the next question.

What is this business about a “sweet savour to the Lord” (Gen 8:21)?

This was not meant in any literal sense, considering that, not being essentially material, God had no need of food, he had no nose, etc. The smell of the sacrifice was probably not unlike that of delicious, savory food; the “sweet savour” thus describes how the sacrifice smelled to those offering the sacrifice. But, regardless of the smells wafting in the air to human noses, not every sacrifice was a “sweet savour,” because sin and trespass offerings did not have such a savor; while God required them, those sacrifices (symbolically, of course) smelled like sin to him. Hence the symbolic meaning of a sacrifice with a “sweet savour” is that the sacrifice was acceptable or welcome to God. This is plausible because Noah’s first activity upon disembarking was to sacrifice—his first thought was of God. It was not the scent of the meat but the performance of faithfulness that God enjoyed, and evidently, Noah was not making a sin offering for dead men; the dead were dead and no sacrifices for them were possible. These sacrifices were on behalf of Noah and his family, the living remnant. Indeed what follows this sweet-smelling sacrifice is “I will not again curse the ground any more for man’s sake” (8:21); and two verses later, beginning at 9:1, God blesses the remnant with man’s first laws and follows that with a covenant of divine peace. All was right with the world once again, for a short time.—Another common observation about “sweet savour” is that the root word rendered “sweet” is Hebrew נִיחוֹחַ, nichoach, glossed by Strong’s as “a quieting, soothing, tranquilizing”; hence it is said that the savor is actually a “savour of rest” (as Henry puts it). In this case, what the sacrifice metaphorically smelled of, to God, was not so much sweetness as restfulness. (It seems the Septuagint used ὀσμὴ εὐωδίας, which is where “sweet savour” came from.) Later uses of the phrase in connection to sacrifice are sometimes held to be allusions to this original “sweet” savor; the sacrifice gives spiritual rest to the penitent as Noah’s landing gave him rest from his yearlong peril. Thus Noah’s journey in the ark was not unlike that of Moses’ with the tabernacle: “My presence shall go with thee, and I will give thee rest.” (Ex 33:14)