As we said before, so say I now again, If any [man] preach any other gospel unto you than that ye have received, let him be accursed.
Note 11 at Ga 1:9: Paul's curse in the previous verse was so strong and so startling that some of the Galatians could have thought, "Surely he couldn't mean what it looks like he means." To leave no doubt, Paul was, in a sense, saying, "I said it once and I will say it again, if anyone preaches a gospel to you that is different, let him be accursed--eternally condemned!" No one could misunderstand that.
Today it is not politically correct to be absolute about anything. Instead, phrases like "I believe," "it's possible," "probably," "most likely," etc., are used. But the Gospel is absolute. "Neither is there salvation in any other: for there is none other name under heaven given among men, whereby we must be saved" (Ac 4:12, emphasis mine). "I am the way, the truth, and the life: no man cometh unto the Father, but by me" (Joh 14:6).
We need to be as sure that we are preaching the true Gospel as Paul was. Then once we have no doubts, we need to be as bold in defending the Gospel as Paul was. "If any man speak, let him speak as the oracles of God" (1Pe 4:11).