And this is the promise (κα αυτη εστιν η επαγγελια). See 1. John 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. John 1:2). In Acts 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 1. John 3:3 by εκεινος.