Addressing the premise of your question: That if the Bible doesn’t say something is true it isn’t true. I guess Airplanes don’t fly then and the Trinity isn’t true.
Secondly, you are not supposed to go by the Bible alone, you are supposed to go by the Traditions of the Apostles (2 The 2:15, 2 The 3:6, 1 Cor 11:2), and obey your bishops (Hebrews 13:17, Acts 20:28, Titus 1:7) and Scriptures (Rom 4:3).
Try again.
This is a complete misunderstanding of Sola Scriptura. Any view looks inane, if it’s not fairly treated.
Sola Scriptura includes a denial that that any other source of teaching is of the same authority or finality - other sources do not have the God-given, God-breathed, inspired authority of the Word of God - which is why in Reformed theology ideas of theologians, the Reformers included, are subject to correction by God Who speaks through the words of Scripture.
That is why there are subordinate standards for faith - such as Confessions of Faith, catechisms, and other works of godly men which have been undertaken to strengthen & nourish the Body of Christ. But useful as they are, they are neither equal to Scripture, nor can they supplement it, nor be equal in honour to it, because they are not inspired, but are the fruits of the grace of the Redeemer in His Church, nothing more.
What is not in Scripture, or at least derivable from it “by good and necessary consequence”, can therefore not be taught as doctrine or enforced as doctrine. So the Fathers, useful as they can be, cannot be heeded when they teach a doctrine which obscures the Gospel in any way - so their sacramental teaching is not acceptable when (say) they witness to more than two sacraments, these being the Lord’s Supper and Baptism - because these two alone are clearly taught in the Word of God.
So infant baptism is defended, even though not mentioned in the Word, because it is derivable from it by good and necessary consequence - for example, certain things
are revealed, & from these follow various conclusions. An example: God works with His Church covenantally - so this fact, attested by Scripture, is put together with others, such as that the children of believing parents are sanctified by them. So children can be in a covenant relationship with God by way of the faith of their parents - baptism is the Scriptural replacement for circumcision, by which infants entered into the old covenant, so, children may lawfully receive baptism. Everything is a revealed Scripture truth, except the conclusion - and as its premises are revealed, so is it.
The same is not true of the Assumption or the Perpetual Virginity - there are pointers to both in Scripture, but nothing more certain. So Calvinists don’t accept either. They respect Mary, but they don’t hold doctrines which could, after all, be completely haywire. The Resurrection is a certainty, because Scripture proclaims it, which means that God does. The Assumption is not in Scripture - therefore God does not Will that it should be believed. How do we know what we are to believe ? Scripture contains it. And the Holy Spirit, the Spirit of Truth by witnessing with our spirits confirms the authority of the Scriptural witness to us.
As for traditions mentioned in the Bible, there is no way to know what they are - it is perfectly possible that they were made into parts of the NT. Unless Catholics are:
a) in a position to know know otherwise
b) able to show
how they know that they know otherwise
- they are expressing a doctrinal position without being able to say how they know both that they are right & that those who disagree are wrong. People cannot say “belief X is in Tradition”, unless they know what the belief is, & whether it is in tradition or not. And to say all that, they have to know those things, otherwise they are not talking with knowledge at all, but saying what means nothing. Otherwise, anything at all can be said to be in Tradition: Highland Dancing, cooking rabbits, computer science.
So far, I haven’t seen a reason why we cannot say that Jesus taught the Apostles how to build aeroplanes. There is no evidence for it - but there is none for a number of theological ideas, which are believed nonetheless. This is back-to-front, as well as unorthodox, because it implies that infallibility can after all
create truths, instead of being a response to them. But that would corrupt the faith. It makes the faith subject to the bright ideas current at the time, instead of being a witness to a determinate, explicit faith. It becomes a nose of wax, as stable as the tides of the sea.
Whatever the uncertainties about Scripture, there are things it cannot mean. Some uncertainties (over Baptism, say) are compatible with Protestant faith; others, are not. ##