Let me answer them slightly out of order as I think that may be more helpful.
WHY is sola Scriptura not valid?
Becaise there is no objective authority or fact that states sola scriptura is taught in Scripture.
What is so important that God left out of Scripture that someone feels they need to add later?
This is a loaded question in that it implies God is careless with his facts (which is not the Catholic claim). But I'll answer it anyway.
We read from Scripture itself the following: John 21:25 But there are also many other things which Jesus did which, if they were written every one, the world itself. I think, would not be able to contain the books that should be written.
This follows: 21:24 This is that disciple who giveth testimony of these things and hath written these things: and we know that his testimony is true.
So the context is clear: verse 25 is clearly saying that there are many other things Jesus did that are *also* helpful so that "we know his testimony is true", that aren't recorded. This obviously necessitates a greater repository of Truth than is contained in the written Word.
What is lacking in it that makes it inadequate for the man of God to be complete?
See above. If you have issue with the fact that everything Jesus did is not in Scripture, take it up with St. John. In addition to this though, no one is saying Scripture doesn't "complete" the man of God. It most certainly does. See below.
WHY isn't the Bible sufficient according to Catholics?
Scripture IS sufficient to "complete" the man of God, but just as any knight isn't "complete" with just a sword, neither is the man of God.
You see, this is the point that every anti-Catholic refuses to acknowledge: Scripture is one part, a vital part yes, but only ONE part of completing the man of God. And there exists no Scripture that says differently (except in the OPINIONS of some men).
Yes it does. It is adequate for the man of God to be wise to salvation, complete, equipped for every good work.
You have to know this one is coming.
2 Timothy 3:14-17 But as for you, continue in what you have learned and have firmly believed, knowing from whom you learned it and how from childhood you have been acquainted with the sacred writings, which are able to make you wise for salvation through faith in Christ Jesus. All Scripture is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness that the man of God may be complete, equipped for every good work.
Complete means complete. Paul says that Scripture is adequate. You say it's not. Guess who I'm throwing my lot in with?
There is also this....
Ephesians 6:10-20 Finally, be strong in the Lord and in the strength of his might. Put on the whole armor of God, that you may be able to stand against the schemes of the devil. For we do not wrestle against flesh and blood, but against the rulers, against the authorities, against the cosmic powers over this present darkness, against the spiritual forces of evil in the heavenly places. Therefore take up the whole armor of God, that you may be able to withstand in the evil day, and having done all, to stand firm. Stand therefore, having fastened on the belt of truth, and having put on the breastplate of righteousness, 15 and, as shoes for your feet, having put on the readiness given by the gospel of peace. In all circumstances take up the shield of faith, with which you can extinguish all the flaming darts of the evil one; and take the helmet of salvation, and the sword of the Spirit, which is the word of God, praying at all times in the Spirit, with all prayer and supplication. To that end keep alert with all perseverance, making supplication for all the saints, and also for me, that words may be given to me in opening my mouth boldly to proclaim the mystery of the gospel, for which I am an ambassador in chains, that I may declare it boldly, as I ought to speak.
And where are the extra-Biblical teachings of Catholicism mentioned in that?
Becaise there is no objective authority or fact that states sola scriptura is taught in Scripture.
Likewise there is no objective source which states that tradition is taught in Scripture because Catholics claim the Catholic church wrote Scripture from oral tradition. That eliminates it by default because if they wrote it, it is not an objective source to use to support the doctrine of tradition.
Congratulations. You just disqualified and invalidated the Catholic church claim for sacred tradition.
We read from Scripture itself the following: John 21:25 But there are also many other things which Jesus did which, if they were written every one, the world itself. I think, would not be able to contain the books that should be written.
This follows: 21:24 This is that disciple who giveth testimony of these things and hath written these things: and we know that his testimony is true.
That is true that Jesus did more than was recorded. So? Did God think we needed it? No, because if you read chapter 20, John also states THIS....
John 20:30-31 Now Jesus did many other signs in the presence of the disciples, which are not written in this book; but these are written so that you may believe that Jesus is the Christ, the Son of God, and that by believing you may have life in his name.
*these things*. The things that John recorded. THOSE were the things written that we may believe in Jesus. That's all we need to know to come to a knowledge of who Jesus is and believe in Him.
Sounds a bit circular to me...
What is so important that God left out of Scripture that someone feels they need to add later?
This is a loaded question in that it implies God is careless with his facts (which is not the Catholic claim). But I'll answer it anyway.
We read from Scripture itself the following: John 21:25 But there are also many other things which Jesus did which, if they were written every one, the world itself. I think, would not be able to contain the books that should be written.
This follows: 21:24 This is that disciple who giveth testimony of these things and hath written these things: and we know that his testimony is true.
So the context is clear: verse 25 is clearly saying that there are many other things Jesus did that are *also* helpful so that "we know his testimony is true", that aren't recorded. This obviously necessitates a greater repository of Truth than is contained in the written Word.
Well; you gave an answer; but not to the question asked...
You can not further nor defend your position by perverting scripture...There is nothing in verse 25 that says the things Jesus did beyond what was written are 'also helpful'...It plain flat out does not say that...And John goes on to say that what was given us is all we need...
If you can't base your position on the truth of the bible, you don't have a position...
That is essentially what the Catholic church is saying.
God didn't include everything we needed to know so here they are, the heroes, providing everything they claim God left out of Scripture.
“But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed. As we said before, so say I now again, If any man preach any other gospel unto you than that ye have received, let him be accursed.” Galatians 1:8-9
Please show where the apostles taught the assumption of Mary and that she was to be considered the “queen of heaven” and co-redemptrix with Christ. If you cannot we shall consider what the Catholic Church teaches to be “another gospel” and accursed as such.
The only source we know to be inspired by the Holy Spirit as to what the apostles taught is scripture.
This is SUCH an absurdity - it is inherent in the contract. You would have us believe that the party of the second part (Man) has authority over the party of the first part (YHWH) in a contract which only YHWH is obligated to perform (Only YHWH walked between the halves). The contract cannot be changed. Ergo, all claimed questions/revisions revert to the founding document. Period.
God doesn't count?