He went to synagogues first in every city he went to and preached there. And they used Scripture.
They were OT Scriptures....does that make them any less worthy or inspired??
So? Is it any less Scripture, Divinely inspired, or valid?
You do realize, that when Jesus quoted Scripture during His temptation by Satan, to validate Himself and His ministry, and on the Road to Emmaus that He used OT Scripture, don't you?
It was that very Scripture that foretold His coming and told people what to look for so they could identify Him.
So here comes Paul along, showing them from Scripture how Jesus fulfilled it and thus demonstrating that fact. That gave his claims credibility.
There's also the complicating factor that the doctrines which Catholicism declares to be *sacred tradition* did not yet exist and the traditions the Jews added to Scripture were roundly condemned by Jesus Himself.