Albert Barnes Commentary 1 John 2:24

"As for you, let that abide in you which ye heard from the beginning. If that which ye heard from the beginning abide in you, ye also shall abide in the Son, and in the Father." — 1 John 2:24 (ASV)

Let that therefore abide in you. Adhere steadfastly to it; let the truth obtain a permanent lodgement in the soul. In view of its great importance, and its influence on your happiness here and hereafter, let it never depart from you.

Which you have heard from the beginning. That is, the same doctrines which you have always been taught regarding the Son of God and the way of salvation (See Barnes on 1 John 2:7).

You also shall continue in the Son, and in the Father. Truly united to the Son and to the Father; or having evidence of the favour and friendship of the Son and the Father.