5 Every word of God is flawless;
he is a shield to those who take refuge in him.
6 Do not add to his words,
or he will rebuke you and prove you a liar. (Prov. 30:5-6, NIV)
How does man add to His words? Throughout history the most common method is to elevate tradition to the level of scripture. In some cases beyond scripture: in his tirade against Luther, the Catholic saint Cajetan argued that the pope was “above scripture”.
Of course, scripture has a different perspective of itself:
I will worship toward Your holy temple,
And praise Your name
For Your lovingkindness and Your truth;
For You have magnified Your word above all Your name. (Ps. 138:2, NKJV).
If scripture is above His name, and the pope is above scripture, well then we have a real problem.
It amazes me that the Catholic Church, in its elevation of tradition, does not see itself making the same mistake as the Pharisees.
But what about…
Therefore, brethren, stand fast and hold the traditions which you were taught, whether by word or our epistle.(2 Th. 2:15, NKJV)
Does this verse not explicitly support the Catholic position? The answer is, of course, absolutely not. Paul is writing to the Thessalonians to correct the effect of a forged letter containing false doctrine (2. Th. 2:2). He is telling them, in effect: Believe only what you heard us preach and what is contained in authenticated epistles. The fact that he refers to these teachings as "traditions" is irrelevant to the Catholic position, in spite of a desperate attempt to cling to one of the very few instances in scripture where the word tradition is used in a positive sense. The traditions spoken of here are scripture itself and self-consistent expositions of scripture, such as apostolic teaching (which was either about scripture or became scripture). This is quite different from the non-scriptural Catholic traditions that at best can be traced back to erroneous teaching of the early (but post apostolic) era, and at worst seem to have coalesced out of the aether as previously undiscovered truth.
Sola Scritura Does not mean that one cannot learn from hearing the word of God in the form of a sermon or a Sunday school. It means that the pastor or priest or teacher should not teach anything with his lips that he cannot substantiate with scripture. In means that if Paul himself were a guest pastor in your church, he probably would give a sermon explaining and clarifying some aspect of scripture, but what he wouldn’t do is add to it.