A.T. Robertson Commentary 1 John 3:19

"Hereby shall we know that we are of the truth, and shall assure our heart before him:" — 1 John 3:19 (ASV)

Shall we know (γνωσομεθα). Future middle indicative of γινωσκω, at any future emergency, we shall come to know by this (εν τουτω) "that we are of the truth" (οτ εκ της αληθειας εσμεν).

Before him (εμπροσθεν αυτου). In the very presence of God we shall have confident assurance (πεισομεν την καρδιαν ημων, either we shall persuade our heart or shall assure our heart) because God understands us.