I have no way of disputing this.
No. It's a matter of faith — Revelation is not 'accessible' to reason, else we would arrive at that data under our own intellectual light.
What did the Apostles say about the Trinity?
Well we can safely assume:
1: They profess a belief in God the Father;
2: They profess a belief in Jesus Christ as Son of God, born as a man, crucified, died and resurrected in the flesh;
3: They profess a belief in the Holy Spirit as truly distinct from the Father and the Son.
Working back:
They believed in Jesus Christ
as a person, all historical argument aside, there are no real grounds for thinking that they did not believe He was man, and he was God.
They were taught to address the Father
as a person, as the very term 'abba' denotes a person, in relation to another person. That is, although the term 'father' can be adduced of a creator, a maker, etc., it does not imply quite the depth of intimate relation that abba implies.
They taught the Holy Spirit as being God, like the Son, but like the Son simultaneously other than God, the Father, and as such
as a person, in that as the Son chose to make himself known as a person, and revealed the Father as a person, then it is fitting to regard the Holy Spirit, in its absolute equality with the Father and the Son, as a person.
That an explicit Trinitarian doctrine is not expressed is simply explained: For one, there was no serious theological dispute about the matter, as there was no such dispute about the Son until the Arian controversy, that called for a more sophisticated doctrine of the Holy Spirit.
Peter says of Cornelius: "Can any man forbid water, that these should not be baptized, who have received the Holy Ghost, as well as we?" (Acts 10:47). Not received Jesus, or even the Spirit of Jesus, but the Holy Spirit as other.
St Paul was preaching the Holy Spirit as distinct and personal from the outset of his ministry — exegetes of the Catholic and Orthodox traditions teach that, and the Anglican exegete Henry Barclay Swete (1835-1917) has shown such by his knowledge of Greek.
"and [that] no man can say that Jesus is the Lord, but by the Holy Ghost" 1 Corinthians 12:3. See also Ephesians 1:13, 3:5, 4:30 and 1 Thessalonians 4:8.
It is accepted that Ephesians, for example, was written after Paul's martyrdom, about 80AD, but by someone thoroughly steeped in Pauline theology. This makes this letter contemporary with the Synoptics.
The Book of Acts, written by Luke about the same time, is called The Gospel of the Holy Spirit — And in Peter's first discourse at Pentecost he says "But Peter said to them: Do penance, and be baptized every one of you in the name of Jesus Christ, for the remission of your sins: and you shall receive the gift of the Holy Ghost" Acts 2:38.
Until then, the disciples had assumed that the return of the Messiah would signal the re-establishment of the Davidic Kingdom — when He told them to wait in Jerusalem, He was asked "Lord, wilt thou at this time restore again the kingdom to Israel?" Acts 1:6.
So Peter's preaching of the Son and the Holy Spirit, as God and yet distinct from the Father and each other, begins after Pentecost — it is a subsequent revelation and not part of Jesus' pre-Ascension teaching, although once realised, the preparation for it can be seen in the teaching of Christ whilst on earth, and foreshadowed in the Hebrew Scriptures.
It's worth noting that many claim the Holy Spirit is an invention of the Johannine Community, whereas the above indicates evidence long before the Gospel of John. The Gospel does however 'explain' something of the silence of the matter in the Synoptics, when he says "Now this he said of the Spirit which they should receive, who believed in him: for as yet the Spirit was not given, because Jesus was not yet glorified" (John 7:39).
It's also worth noting that anything we say about the Trinity can only be analogous, as we cannot conceive of the Trinity, any more than we can know what it's like to be God. Thus even the concept of 'person' is analogous in that sense, but being as Christ used it, if it's good enough for Him, it's good enough for us, and the idea of person expresses in a way that no other term can, the degree of intimacy of union spoken of in the Christian Tradition.
Thomas