This is a good question. I am not sure I have the answer but I do think I am on the right trail.
5 Remember ye not, that, when I was yet with you, I told you these things?
(So we know that Paul had taught them about this before. My guess is that he taught them from Daniel and interpreted at length so that THEY KNEW what he was talking about. Unfortunately we were not there and do not KNOW what he had told them. Nevertheless it is probable that the references in the following three verses clues us in as to WHAT in Daniel he would have been teaching. ..
.
6 And now ye know what withholdeth that he might be revealed in his time.
7 For the mystery of iniquity doth already work: only he who now letteth
will let, until he be taken out of the way.
8 And then shall that Wicked be revealed, .....
I think one of the biggest clues are "Mystery of iniquity" = Daniel 8
23 And in the latter time of their kingdom, when the transgressors are come to the full, a king of fierce countenance, and understanding dark sentences, shall stand up.
24 And his power shall be mighty, but not by his own power: and he shall destroy wonderfully, and shall prosper, and practise, and shall destroy the mighty and the holy people.
And also the reference to he that holds back could be referencing Dan 12:
1And at that time shall Michael stand up, the great prince which standeth for the children of thy people: and there shall be a time of trouble, such as never was since there was a nation
even to that same time: and at that time thy people shall be delivered, every one that shall be found written in the book.
My guess is that Paul had taught them previously an interpretation from Daniel concerning these prophecies and that they KNEW he was talking about Dan 12 and probably something that Michael was going to hold back until he stands up.
He must have been talking about Daniel and so that is where I would go for the answers. I think that if we can come up with an interpretation about "He that withholdeth, and he that letteth will let until he be taken out of the way" from the book of Daniel we will have discovered WHAT "I told you these things" was that he had previously taught them. If we come up with an interpretation that can not be found in the book of Daniel we are probably not teaching what Paul had previously taught them.