'Knowing' in this passage is referring to the spiritual gift of words of knowledge (v8), not a general knowledge of God. The revelatory knowledge that gift brought was "in part" until "completeness" came after which we had full knowledge of God's revelation to man. We will never possess perfect general knowledge as God does, even in Heaven.
Paul knew he was writing scripture, and knew that scripture could only be written by an Apostle of Christ (or someone under their authority). He knew that the work of the apostles make up the foundation of the church (Eph 2:20) and one day that foundation would be complete. The councils only met to decide which books had apostolic authorship and which didn't.
"Completeness" (as the NIV and others versions renders it) is a better translation of
Mounce Lexicon:
τέλειος (teleios)
brought to completion; fully accomplished, fully developed, Jas. 1:4a; fully realized, thorough, 1 Jn. 4:18; complete, entire, as opposed to what is partial and limited, 1 Cor. 13:10; full grown of ripe age, 1 Cor. 14:20; Eph. 4:13; Heb. 5:14; fully accomplished in Christian enlightenment, 1 Cor. 2:6; Phil. 3:15; Col. 1:28; perfect in some point of character, without shortcoming in respect of a certain standard, Mt. 5:48; 19:21; Col. 4:12; Jas. 1:4b; 3:2; perfect, consummate, Rom. 12:2; Jas. 1:17, 25; compar. of higher excellence and efficiency, Heb. 9:11
Friberg Lexicon:
26442 τέλειος, εία, ον complete, perfect; (1) with its chief component as totality, as opposed to partial or limited; (a) of thingsin full measure, undivided, complete, entire (RO 12.2); substantivally τὸ τέλειον the finish, completeness (1C 13.10); comparative τελειότερος, τέρα, ον more complete or perfect (HE 9.11); (b) of persons complete, perfect (MT 5.48; 19.21); (2) with its chief component being full development as opposed to immaturity; (a) of persons full grown, mature (1C 14.20); substantivally οἱ τέλειοι adults, mature persons; used of spiritually mature persons (1C 2.6); (b) of things fully developed, complete (JA 1.4; 1J 4.18); (3) with its chief component being full preparation or readiness complete, perfect (CO 1.28; JA 3.2); in all its meanings τ. carries the component of a purpose that has been achieved
Gingrich Lexicon:
6356 τέλειος
τέλειος, α, ον having attained the end or purpose, complete, perfect—1. of things Js 1:4a, 17, 25; Hb 9:11; 1 J 4:18. τὸ τέλειονwhat is perfect Ro 12:2; 1 Cor 13:10.—2. of persons—a. full-grown, mature, adult adj. 1 Cor 14:20; Eph 4:13; subst. Hb 5:14. For 1 Cor 2:6 the sense may be adult, or it may belong under b below.—b. the initiate into mystic rites, perh. 1 Cor 2:6 (see a above); probably Phil 3:15; Col 1:28.—c. perfect, fully devel
UBS Lexicon:
6023 τέλειος , α , ον complete, perfect, whole ( ἔργον τ. full effect, successful results Jas 1.4); full-grown, mature (of persons); τελειότερος more perfect (He 9.11)
LEH Lexicon:
8823 τέλειος
τέλειος,-α,-ον+ - A 3-9-1-4-2-19
Gn 6,9; Ex 12,5; Dt 18,13; JgsB 20,26; 21,4
perfect, entire, without spot or blemish (of sacrificial victims) Ex 12,5; perfect (in his kind; of pers.) Gn 6,9; perfect, complete, expert 1 Chr 25,8; complete Jer 13,19; absolute Ps 138(139),22
Cf. DANIEL, S. 1966, 287-288.295-296; WEVERS 1993, 81; ïNIDNTT; TW
LSJ Lexicon (Abridged):
42280
τέλειος and τέλεος, α (Ion. η) , ον, in Att. also ος, ον: (τέλος):-having reached its end, finished, complete, Il., etc.: