A.T. Robertson Commentary 1 John 2:25

"And this is the promise which he promised us, [even] the life eternal." — 1 John 2:25 (ASV)

And this is the promise (κα αυτη εστιν η επαγγελια). See 1:5 for the same idiom with αγγελια (message). This is the only instance of επαγγελια in the Johannine writings. Here "the promise" is explained to be "the life eternal" (1:2). In Ac 1:4 the word is used for the coming of the Holy Spirit.

He promised (αυτος επηγγειλατο). First aorist middle indicative of επαγγελλω. Αυτος (he) is Christ as is seen in 3:3 by εκεινος.