The Greek word translated "traditions" is "paradoseis," which literally means "things handed over" or "passed down." There were many traditions of men that were based upon error and false theories. These are heartily condemned in the Bible because they contradict or subvert the truth of God (Matthew 15:3).
But these traditions spoken of by Paul in 2 Thessalonians 2:15 were those things that had been handed over by Christ and that the apostles were now passing along to the world. Paul does not give a wholesale endorsement to tradition as such. He limits the traditions to which they should "hold fast" to those the church had received "by word or our epistle." This encompassed only the inspired teaching or writing of those who had received their message directly from Christ and were ministering to the churches at that time. Paul declared that if any person or angel should preach any other gospel than the one he had preached, "let him be accursed." Galatians 1:8