The Bible does contrast believers being in the truth (being in the Truth, Christ, and the body of sound doctrine) vs being in darkness, error, etc.
Jn. 14:6 does show that we are saved by a person, not a church or organization.This is why one can be saved by faith in Christ, not by theological excellence or conformity/uniformity with a fallible organizations like Catholic church (Popery), LDS/Mormon (modern 'prophets'), or JW/WT/GB. Evangelicals do not have a pope, prophet, or GB. Our faith is based on Sola Scripture (Reformational Scripture alone; properly translated/interpreted/applied).
If JWs had truth, they could claim to be in the truth without denying Jesus, the Truth, even if they are not infallible (like the early church which also had doctrinal disputes, divisions, etc.). If evangelicals are on the right track, they also can follow the person of Christ as supreme truth and claim to be in the truth vs in darkness (worshipping false idols, gods, false teachings, etc.). The Bible is not afraid to emphasize sound doctrine and warn about false doctrine, so neither should we be afraid to defend and proclaim truth (which does not mean we are infallible on every point, but at least right on core, essential truth like the doctrine of God/Christology/soteriology=salvation).