There is a strong parallel phrase between the Didache (50-70 AD) and the Epistle of Barnabas (90-131 AD), two very early Christian writings found in the modern collection known as the Apostolic Fathers.
In anticipation of the last days and the return of Jesus, the Didache emphasizes the necessity of being found perfect or complete in the last days, otherwise our past faith will “be of no use.”
Watch over your life: do not let your lamps go out, and do not be unprepared, but be ready, for you do not know the hour when our Lord is coming. Gather together frequently, seeking the things that benefit your souls, for all the time you have believed will be of no use to you if you are not found perfect in the last time. (Didache 16. 1,2; Holmes, 3rd Ed.)
In the 1912 Loeb edition, Kirsopp Lake translates the last passage as, “for the whole time of your faith shall not profit you except ye be found perfect at the last time.”
Similar language is used in the Epistle of Barnabas when talking of the last days:
Consequently, let us be on guard in the last days, for the whole time of our faith will do us no good unless now, in the age of lawlessness, we also resist, as befits God’s children, the coming stumbling blocks, lest the black one find an opportunity to sneak in. (Barnabas 4. 9b; Holmes, 3rd Ed.)
Both are directly speaking of the importance of continued spiritual soberness in the last days. The Greek has almost identical wording in both passages where it says “for the whole time of your/our faith will do you/us no good unless [now] in the last/lawless time […].”
Didache: οὐ γὰρ ὠφελήσει ὑμᾶς ὁ πᾶς χρόνος τῆς πίστεως ὑμῶν, ἐὰν μὴ ἐν τῷ ἐσχάτῳ καιρῷ […]
Barnabas: οὐδὲν γὰρ ὠφελήσει ἡμᾶς ὁ πᾶς χρόνος τῆς πίστεως ἡμῶν, ἐὰν μὴ νῦν ἐν τῷ ἀνόμῳ καιρῷ […]
There is no doubt that there either was a common source that both quoted from, or Barnabas borrowed here from the Didache. Regardless, the teaching that we need to be found faithful at the end was a common teaching present among the earliest believers. The eternal state of a Christian was not strictly viewed only as a simple profession of past faith, but in terms of continued faithfulness and endurance up until the end of our life.
Eventually I may take the time and effort to complete a comprehensive survey of this teaching in the earliest non-canonical Christian writings, but until then this is just one morsel of early Christian thought to chew on.