The clearest antithesis is in stanza 2:
Will it pull you forward
Or push you back in fear?
Why it qualifies — test it against the book’s own definition.
A second, looser antithesis runs through stanza 3:
There is such ease in comfort / To maintain the status quo,
But this isn’t what we are made for / This isn’t how we grow.
Here the contrast is comfort against growth, held in place by the pivot word But and by the repeated “This isn’t”. The structure is less tightly parallel than the pull/push pair, but the opposition is the same kind: two states set against each other so that choosing one means rejecting the other.
Why antithesis suits this poem particularly well. The poem is about a moment of decision, and a decision is by nature a choice between two things. Antithesis is the grammar of decision-making — it removes the middle ground and lays the alternatives side by side, which is exactly what a person standing at the foot of the stairs experiences. Compare Armstrong’s sentence: he too was describing a single step, and he too needed two opposed phrases to say what it meant.
One difference from the book’s examples. “Speech is silver, but silence is gold” and “Man proposes, God disposes” are statements — the contrast is settled, and the speaker tells you which side wins. Langley’s antithesis is a question: “Will it pull you forward / Or push you back in fear?” Nothing is settled, and no answer is given. The device is the same; the effect is opposite, because the choice is left with the reader.