The questions you need to answer are all one step beyond that.
- If sola scriptura were true, why does ANYONE need a catechism? - Which by the way is the meaning of "tradition" - paradosis, the entire faith handed down. It is not the colloquial usage now. Like it or not ALL view scripture through a lens of tradition, whether they recognise it or not. The catechisms etc are that lens.
- The catechisms and confessions mandate specific interpretation of scripture. How can you know that that version is true?
- By what power and authority are catechisms written and decisions taken?
- How can that power be traced back to pre new testament christianity. And where is the continuity?
Hint. Consider the power to "bind and loose". Who has it, and when was it used.
Take a simple example. Either the only reference to "keys of the kingdom" is the OT , herald the same office of steward, when Jesus referred to it in respect of Peter, or it does not. By what authority do you say that is true or not?
The power given to peter "bind and loose" which means give definitive judgement on doctrineand later (jointly) to other apostles gives the ability to resolve matters then "bound in heaven or not". Where is the power to give definitive judgement now?
When scripture refers to the church as the "foundation of truth" what does it mean.
When yet another branch of yet another denomination fractures into two , because of a point of doctrine or teaching, by what authority do they say the "other lot" are wrong?
The question of authority is what divides christians.
The catechisms confessions, statements of faith, call them what you wil, prove tradition
And the believe in veracity of a catechism, is also declaring authority to say it is true.
But how do you know?
When at a council the fatherspresent at Chalcedon and the "Tome of Leo" said "there speaks peter" what did they mean? When those from Iraneus, to Augustine list popes, (in augustines case as a auhtority against donatism) why did they do that?
Your choice. But those are the questions.
Christianity did not start at the reformation.
Because they all disagree, Only one of those catechisms can be truth. The rest are partially false. How so if they have authority?
Those are the questions. All must decide answers for themselves.
What underpins their belief in veracity of a catechism, confession, statement of faith , call it what you will.