[BenghaziMemoriam]

If your point is true that not all Christian teachings were not completed with the apostles, how could the first century believer know what was necessary for salvation? Or did they have sufficient information at that time?
And why should any current Christian be held accountable for beliefs the early Christians were not taught by the apostles?

In any age, it would seem that there are certain bare basics that are required to be taught to (and accepted by) the believer. The Apostles' Creed seems to fit that (by definition). However, there's more than just knowing and believing:

"What good does it do to speak learnedly about the Trinity if, lacking humility, you displease the Trinity? Indeed it is not learning that makes a man holy and just, but a virtuous life makes him pleasing to God. I would rather feel contrition than know how to define it. For what would it profit us to know the whole Bible by heart and the principles of all the philosophers if we live without grace and the love of God? Vanity of vanities and all is vanity, except to love God and serve Him alone."
- Thomas à Kempis, The Imitation of Christ
https://ccel.org/ccel/kempis/imitation/imitation.ONE.1.html

"To whom much is given, much will be required" (Luke 12:48).

[kosciusko51]

So, what teachings necessary for our salvation and spiritual life are not taught either explicitly or implicitly in Scripture? And how do you prove that?