The apostles of Christ taught that their teachings
(New Testament writings) were all that was necessary for us to understand and know the will of
God, be saved, and live in a manner pleasing unto Him:
John 20:30-31... "Therefore many other signs Jesus also performed in the presence of the disciples, which are not written in this book; but these have been written so that you may believe that Jesus is Christ, the Son of God; and that believing you may have life in His name." Here John tells us that the things which he wrote are sufficient for those who read it to "believe" in Jesus and to "have life" - eternal life, salvation - "in His name."
2 Peter 1:2-3... "Grace and peace be multiplied to you in the
knowledge of God and of Jesus our Lord; seeing that His divine power has granted
to us everything pertaining to life and godliness, through the true knowledge of
Him who called us by His own glory and excellence."
Ephesians 3:3-5... In verses 3 and 5, the apostle Paul writes, "...that by revelation there was made known to me the mystery... which in other generations was not made known to the sons of men, as it has now been revealed to His holy apostles and prophets in the Spirit." In the middle of this, he assures us in verse 4 that this is both available and understandable to us, by way of the things he wrote: "By referring to this (the revelation to Paul), when you read you can understand my insight into the mystery of Christ."
2 Timothy 3:16-17... "All scripture is inspired by God and profitable
for teaching, for reproof, for correction, for training in righteousness; so that
the man of God may be adequate, equipped for every good work."