A.T. Robertson Commentary Hebrews 3:13

"but exhort one another day by day, so long as it is called To-day; lest any one of you be hardened by the deceitfulness of sin:" — Hebrews 3:13 (ASV)

So long as it is called today (αχρις ου το σημερον καλειτα). The only instance in the N.T. of this conjunction (αχρ or αχρις or αχρις ου, etc.) with the present indicative in the sense of "so long as" or "while" like εως. Elsewhere it means "until" and with either the aorist indicative (Acts 7:18), the future (Revelation 17:17), or the aorist subjunctive (Revelation 7:3).

Lest any one of you be hardened (ινα μη σκληρυνθη τις εξ υμων). Negative purpose clause with ινα μη (that not) and the first aorist passive subjunctive of σκληρυνω, the vivid verb from verse 8.

By the deceitfulness of sin (απατη της αμαρτιας). Instrumental case απατη (trick, fraud) as is always the case with sin (Romans 7:11; 2 Thessalonians 2:10). Apostasy (12:4) is their peril and it is a trick of sin.