I believe that Scripture is sufficient as the sum of God's revelation for us today. I've often used 2 Peter 1:3 to defend this doctrine. But lately I've wondered if doing so isn't some warmed-over proof-texting.
Where, exactly, is the Word of God in the context? To say that Jesus Christ has given us all things that pertain to life and godliness through the knowledge of the Father does not prove that all those things are contained in the Word of God.
It is clear in the NT that the Word of God is not all that God has "given us":
". . . who also sealed us and gave us the Spirit in our hearts as a pledge." (2 Cor. 1:22)
"Now may our Lord Jesus Christ Himself and God our Father, who has loved us and given us eternal comfort and good hope by grace, . . ." (2 Thess. 2:16)
"For God has not given us a spirit of timidity, but of power and love and discipline." (2 Tim. 1:7)
"Who has saved us and called us with a holy calling, not according to our works, but according to His own purpose and grace which was granted us in Christ Jesus from all eternity, . . ." (2 Tim. 1:9)
"The one who keeps His commandments abides in Him, and He in him. We know by this that He abides in us, by the Spirit whom He has given us." (1 John 3:24)
"By this we know that we abide in Him and He in us, because He has given us of His Spirit." (1 John 4:13)
"And we know that the Son of God has come, and has given us understanding so that we may know Him who is true; and we are in Him who is true, in His Son Jesus Christ. This is the true God and eternal life." (1 John 5:20)
My purpose is not to argue that Scripture is insufficient for our life and godliness, but to demonstrate that we're on shaky exegetical ground to insist that this passages teaches that truth exclusively. I simply don't think the sufficiency of Scripture is the primary point of this passage. It seems to me to be about the sufficiency of the power of Christ.