How do we know the serpent in Genesis was Satan?

What the text says

• Genesis withholds the name on purpose Calvin’s explanation is better than treating the silence as a problem: the Holy Spirit then purposely used obscure figures, because it was fitting that full and clear light should be reserved for the kingdom of Christ. The Old Testament is deliberately dim here and the New Testament turns the light up. That is not a contradiction; it is the design of the book.
• Revelation names him — and cites Eden doing it That ancient serpent, who is called the devil and Satan (Revelation 12:9; Revelation 20:2). The Greek is ho ophis ho archaios, the serpent of old. John is not coining an image; he is pointing back to a serpent his readers already know, and identifying him.
• Jesus dates the devil’s career to the beginning He was a murderer from the beginning, and does not stand in the truth… he is a liar and the father of lies (John 8:44). The first lie in the world was told in Eden — you will not surely die (Genesis 3:4) — and the first murder followed immediately after. John says the same: the devil has been sinning from the beginning (1 John 3:8).
• Paul assumes it without needing to argue it The serpent deceived Eve by his cunning (2 Corinthians 11:3) sits eleven verses from Satan disguises himself as an angel of light (2 Corinthians 11:14) — same chapter, same argument, same adversary. Paul expects the Corinthians to already know who the serpent was.
• Romans 16:20 is Genesis 3:15 with the name supplied The God of peace will soon crush Satan under your feet. The promise was made about the woman’s offspring and the serpent’s head; Paul applies it to the church and calls the serpent Satan, without argument, as settled fact.
• The creature behaves like no animal It speaks. It knows God’s command. It misquotes it deliberately — did God actually say, ‘You shall not eat of any tree in the garden’? (Genesis 3:1), which is not what God said. It flatly contradicts God, and it supplies a motive: you will be like God. Genesis calls it arum, crafty — a word for moral shrewdness, not animal instinct.
• The sentence is aimed past the animal God curses the serpent to the dust, and then promises enmity between its offspring and the woman’s, and a crushed head (Genesis 3:14-15). Snakes and people have not been locked in a war over the salvation of the world. The curse addresses the animal and the power that used it.
• This is Scripture interpreting Scripture The rule is that the clear passage governs the obscure one. Genesis 3 is deliberately obscure; Revelation 12 is bright daylight. Reading the first by the last is not importing an idea into the text — it is letting the Bible’s own closing word explain its opening one.
• Two supports we would not lean on Isaiah 14:12-15 and Ezekiel 28:11-19 are often used as accounts of Satan’s fall, and Ezekiel does say you were in Eden, the garden of God and speaks of an anointed cherub. But both oracles are addressed to human kings — Babylon and Tyre — in their own setting, and neither mentions the serpent. They may carry an overtone; they cannot carry the argument, and the case above does not need them.

How others have answered

Named only where the words were written about this passage. Quoted phrases are theirs; the summary around them is ours.

• John Calvin (Commentary on Genesis) Holds both halves together: a real animal, and a real spirit using it. The innate subtlety of the serpent did not prevent Satan from making use of the animal for the purpose of effecting the destruction of man. On why Moses does not say so outright, he gives the answer quoted above — the obscurity is deliberate, and the clear light was reserved for Christ’s kingdom.
• Matthew Henry (1662–1714) Blunter: the tempter was the devil, in the shape and likeness of a serpent. He draws the practical warning out of the choice of instrument — Satan’s policy to send temptations by hands we do not suspect, and by those that have most influence upon us.
• Before the New Testament was written The identification is not a Christian innovation read back into Genesis. The Wisdom of Solomon, a Jewish book from the couple of centuries before Christ, already says that through the devil’s envy death entered the world. We do not receive that book as Scripture and do not argue from it — but it shows Jewish readers had drawn the same conclusion from Genesis before any apostle wrote.

Where it lands

The identification is not trivia, because of what is promised in the same breath. Genesis 3:15 is the first sentence of the gospel: the offspring of the woman will crush the serpent’s head, and be wounded doing it. If the serpent is only a snake, that promise shrinks to pest control. If the serpent is the devil, the promise is the whole Bible in one line.

Calvin reads the seed collectively — the human race, which Satan was endeavoring to oppress, would at length be victorious — while granting that since not all humanity defeats Satan, the victory belongs to Christ as head of His church and reaches us through Him. Which is exactly how Paul uses it: God will crush Satan under your feet, church, because He was first crushed under His.

The head wound was given at a cross, by a man born of a woman (Galatians 4:4). The reason the Son of God appeared was to destroy the works of the devil (1 John 3:8). The serpent got a heel; he lost his head.