Hebrews 3:13

While it is called today [α ρ χ ι ς ο υ τ ο σ η μ ε ρ ο ν κ α λ ε ι τ α ι]. Lit. so long as the today is being named. The article points to the former expression - the "today" of ver. 7. It is the day of grace, while salvation through Christ is still attainable.

Through the deceitfulness of sin [α π α τ η τ η ς α μ α ρ ι α ς]. jApath is rather a trick, stratagem, deceit, than the quality of deceitfulness. The warning is against being hardened by a trick which their sin may play them. Note the article, the or his sin - the sin of departing from the living God. The particular deceit in this case would be the illusion of faithfulness to the past.