Speaking as one who hasn't decided which communion (RC or Orthodox) to join yet, what got me to question the doctrine of Sola Scriptura was what some have pointed out already - that is, Scripture itself appears to have a strong traditional element to it. Point out that there are no verses which spell out the table of contents of the Bible. Cite 2 Thess. 2:15. (There is another verse where Paul speaks of "holding fast to tradition" that I can't recall the reference to right now. I think it's in 1 or 2 Corinthians.....?)
Point out that every time that they pick up their Bible, they are relying on the tradition of the Church to tell them which books are, in fact, Scripture. Ask them how they know which books are supposed to be in the Bible. Unless they sat down somewhere and sorted through thousands of ancient manuscripts and decided themselves. In which case, they've got a whole different set of problems.
Also, point out other traditional aspects of their doctrine. If they believe in the Trinity, point out that nowhere does the Bible spell out that particular doctrine. Finally, point out that Scripture itself does not have a verse that says that Scripture is the final authority. Cite the verse that says the church is the "pillar and ground of truth."
I do not agree with you on the trinity in scriptures. THe Bible clearly teaches that the Father is God, that the Son is God and that the Holy Spirit is God. The Bible also clearly teaches that all 3 persones are distincts from one another. And the Bible clearly teaches there is only one God. So here you have the Trinity clearly taught in scriptures.
This argument is kind of popular, but is totaly untrue and foreign to what the Fathers have taught:
Pay attention, therefore, to what I shall record out of the holy Scriptures, which do not need to be expounded, but only listened to.
ANF: Vol. I, Dialogue of Justin, Chapter 55.
Since, therefore, the entire Scriptures, the prophets, and the Gospels, can be clearly, unambiguously, and harmoniously understood by all
, although all do not believe them; and since they proclaim that one only God, to the exclusion of all others, formed all things by His word, whether visible or invisible, heavenly or earthly, in the water or under the earth, as I have shown from the very words of Scripture
St Irenaeus Vol. I, Against Heresies, 2:27:2.
. Therefore since in the earlier books we proved the Divinity of our Lord Jesus Christ while He was in the flesh by the evidence not only of prophets and apostles, but of evangelists and angels as well
, let us now show that He who was born in the flesh was God even before His Incarnation; that you may understand by the harmony and concord of the evidences from the sacred Scripture
St John Cassian On the Incarnation, Book IV, Chapter 1
But why should I maintain the unity of the Name by arguments, when there is the plain testimony of the Divine Voice that the Name of the Father, the Son, and the Holy Spirit is one? For it is written: "Go, baptize all nations in the Name of the Father, and of the Son, and of the Holy Spirit." [Matthew 28:19]
He said, "in the Name," not "in the Names." So, then, the Name of the Father is not one, that of the Son another, and that of the Holy Spirit another, for God is one; the Names are not more than one, for there are not two Gods, or three Gods.
St Ambrose, On the Holy Spirit, Book I, Chapter 13, Section 132
The object of the apostle in thus writing was not to introduce the diversity of nature, but to exhibit the notion of Father and of Son as unconfounded. That the phrases are not opposed to one another and do not, like squadrons in war marshalled one against another, bring the natures to which they are applied into mutual conflict, is perfectly plain from the passage in question. The blessed Paul brings both phrases to bear upon one and the same subject, in the words "of him and through him and to him are all things." [Romans 11:36] That this plainly refers to the Lord will be admitted even by a reader paying but small attention to the meaning of the words.
St Basil the Great, Of the Holy Spirit, Chapter 5, Section 7For to whom is not the gospel plain?
Who is it that hears, "Blessed are the meek; blessed are the merciful; blessed are the pure in heart," and such things as these, and needs a teacher in order to understand any of the things spoken?
St Chrysostom, Four Discourses on the Rich Man and Lazarus, Discourse 3, Section 3
And Trypho said, “Prove now that this is the case, that we also may agree with you. For we do not understand you to affirm that He has done or said anything contrary to the will of the Maker of all things.”
Then I said, “The Scripture just quoted by me will make this plain to you.
It is thus: ‘The sun was risen on the earth, and Lot entered into Segor (Zoar); and the Lord rained on Sodom sulphur and fire from the Lord out of heaven, and overthrew these cities and all the neighbourhood.’ ” [Gen. xix. 23.]
St Justin Martyr, Dialog with Trypho, Chapter 56
So to say the Trinity is not clear in scriptures is untrue. That the formulation of the Trinity is unclear, and rather implicit, that i agree.