A.T. Robertson Commentary 2 Timothy 1:14

"That good thing which was committed unto [thee] guard through the Holy Spirit which dwelleth in us." — 2 Timothy 1:14 (ASV)

That good thing which was committed unto thee (την καλην παραθηκην). Simply, "the good deposit."

Guard (φυλαξον). As in 1 Timothy 6:20. God has also made an investment in Timothy (cf. verse 12). Timothy must not let that fail.

Which dwelleth in us (του ενοικουντος εν ημιν). It is only through the Holy Spirit that Timothy or any of us can guard God's deposit with us.