Message (aggelia). In N.T. only here and 1Jo 1:5, but epaggelia (promise) fifty-one times.
From the beginning (ap' archēs). See 1Jo 1:1 for this phrase and 1Jo 2:7 for the idea. They had the message of love for the brotherhood from the beginning of the gospel and it goes back to the time of Cain and Abel (1Jo 3:12).
That we should love one another (hina agapōmen allēlous). Sub-final clause (content of the aggelia) with hina and present active subjunctive. John repeats the message of 1Jo 2:7.