A mortar and pestle is a tool used to crush, grind, and mix solid substances. The pestle is a heavy club-shaped object and the mortar is a bowl, typically made of hard wood, ceramic or stone. Inside the mortar is a substance to be ground down by the pestle.
I am the mortar. This sinful shell of a body is a vessel. Once, it was a vessel of satan's, his playground of sin in me. Then Jesus's grace saved me and He sent the Holy Spirit to indwell this poor vessel. The Spirit aids me in resisting sin, which this poor vessel stillcontains. This mortar still contains the substance of sin. It is unwanted material.
The Spirit is the Pestle. He is the mechanism that grinds down that sin. (John 16:8). O, Holy Spirit, my Lord, My God! Grind down the sin in me today, crush it into nothing! Please make more room in me for growth in Christ-likeness! (Gal 4:19, Rom 12:2, Phil 2:3-5)