Channa became an arahat after using the knife.
The Book of the Six Sense Bases )
p. 1406 connected discourses bhikkhu bodhi
52 Spk: Channa replied to Sariputtas questions by ascribing
arahantship to himself, but Sariputta, while knowing that
he was still a worldling, just kept quiet. Mahacunda, however,
gave him an exhortation to convince him of this.
53 This “teaching of the Blessed One” is at Ud 81,&10. Spk
explains the connection between the teaching and the present
situation thus: For one who is dependent (nissitassa):
“dependent” on account of craving, conceit, and views; there
is wavering (calitam): palpitation. As Channa is unable to
endure the arisen pain, there is now the palpitation of one
who isn’t free from the grip of such thoughts as “I am in
pain, the pain is mine.” By this, he is telling him, “You’re still
a worldling.” No inclination (natr): no inclination of craving.
No coming by way of rebirth, no going by way of death. This
itself is the end of su#ering: this itself is the end, the termination,
the limit, of the suffering of defilements and of the suffering
of the round.54 Spk: He cut his jugular vein and just then the fear of death
entered him. As the sign of his rebirth destiny appeared, he
realized he was still a worldling and his mind became agi-
tated. He set up insight, discerned the formations, and
reaching arahantship, he attained final Nibbana as a
“same-header” (samasisi; see I, n. 312).
55 Spk: Although this declaration (of blamelessness) was
made while Channa was still a worldling, as his attainment
of final Nibbina followed immediately, the Buddha
answered by referring to that very declaration