Charles Spurgeon Commentary 1 John 2:21-25

"I have not written unto you because ye know not the truth, but because ye know it, and because no lie is of the truth. Who is the liar but he that denieth that Jesus is the Christ? This is the antichrist, [even] he that denieth the Father and the Son. Whosoever denieth the Son, the same hath not the Father: he that confesseth the Son hath the Father also. 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. And this is the promise which he promised us, [even] the life eternal." — 1 John 2:21-25 (ASV)

I have not written to you because you do not know the truth, but because you know it, and that no lie is of the truth. Who is a liar but he that denies that Jesus is the Christ? He is antichrist, that denies the Father and the Son. Whoever denies the Son, the same does not have the Father: [but] he that acknowledges the Son has the Father also. Let that therefore abide in you, which you have heard from the beginning. If that which you have heard from the beginning shall remain in you, you also shall continue in the Son, and in the Father. And this is the promise that he has promised us, even eternal life.

Not transient life, but eternal life, is the great promise of the covenant of grace, and abiding in Christ we possess it.