The only Bible writer to use the word “antichrist” is the apostle John. Note these words in the first letter bearing his name:

“Young children, it is the last hour, and just as you have heard that the antichrist is coming, even now many antichrists have appeared, from which fact we know that it is the last hour. They went out from us, but they were not of our sort . . . Who is the liar but the one who denies that Jesus is the Christ? This is the antichrist, the one who denies the Father and the Son.”—1 John 2:18, 19, 22.

Therefore, the Anti-Christ would come up with a Universal teaching that opposes Jesus as the Christ, the son of the living God. (Mathhew 16:16)

A good numbr of trinitarians believe that the trinity is inspired of God. Ask, what scripture is the trinity in. Most will throw up several scriptures. Usually something... well anything to do with (3). You can easily compare their trinity doctrine to scripture. If the person refuses to show his trinity doctrine, then you are dealing with someone who is decptive and only seeking to stumble you. So please be careful.

Jesus Christ stated that in the time of the end, "The Son of man will send his angels, and they will collect out from his Kingdom all things that cause stumbling and people who practice lawlessness, and they will pitch them into the fiery furnace. There is where their weeping and the gnashing of their teeth will be." (Matthew 13:41-42)

So time is of the essence... it will take some work since most tend to be brainwashed in seeing things that are not really in the Bible. The trinity replaces Jesus Christ for salvation, so don't be surprised when they reject the blood and body of Christ. Simply find some good info on exposing the pagan doctrine.

“[Man of sin] Who opposeth and exalteth himself above all that is called God, or that is worshipped; so that he as God sitteth in [Greek = as] the temple of God, shewing himself that he is God.” (2 Thessalonians 2:4) (KJV 1900)

Instead of the man of sin sitting in the temple of God, he would sit as the temple of God. Which of course is what the Papacy claims for itself. The Papacy claims to be the one true church. The Church = God's temple in the NT.

Says Augustine; "Antichrist means not the prince himself alone, but his whole body, that is, the mass of men who adhere to him, along with him their prince; and they also think that we should render the Greek more exactly were we to read, not “in the temple of God,” but “for” or “as the temple of God,” as if he himself were the temple of God, the Church.1405"

NPNF1-02. St. Augustine's City of God and Christian Doctrine - Christian Classics Ethereal Library

Most Reformation era churches claimed the Papacy is Antichrist including many post reformation churches including early Baptists and Methodists.