Hello, @allegra.brown. I was able to encounter a Korean missionary from their group before. I encourage you to ask specific questions or arguments which you find hard to refute. This will make it easier for us to help you on how to address their arguments.
I remember two arguments when I encountered their korean missionary before:
The first one is in Genesis 1:26, "Then God said, “Let us make man in our image, after our likeness…” He told me that since God said “let us” then it’s more than one. There must be a mother. They will then cite verse 27 saying “in the image of God he created him; male and female He created them.” If we look at the whole passage in v. 27, we could see that “God created man in HIS OWN IMAGE, in the image of God he created him, male and female he created them.” The passage itself is not talking about a separate being which the image of this other being is where man is derived from.
If you talk about the plurality aspect (the “let us” part), and say that this is the Trinity, in my experience, they will ask you about the name of each member of the Trinity in order to say that you don’t know what you’re talking about (their red-herring). So they will cite Matthew 28:19, and say that since the passage talks about the “name” of the Father, Son, and the Holy Spirit, then each of them has a name. Then they will say, “what’s the name of the Holy Spirit”?
The Greek word of name is onoma, which means reputation or character in the Strong’s dictionary. In HELPS Word-studies, it means manifestation or revelation of someone’s character. It’s not a literal name. So their red-herring argument is wrong. If you can’t answer them, they will use a person’s ignorance to say that if you go with them, they’ll reveal to you the “name” of the Holy Spirit.
My suggestion for you is to stay talking about the Bible. If they cite scriptures, then tell them the context of the passage so that you can expose how they are misusing it. Like if they say that they are the only people who are saved, since they are the only ones who keep the passover/Lord’s supper, ask them about the thief on the cross being saved without keeping it, or about Judas who kept it who was not saved?
Or you can engage them about passages that show that we are saved by grace through faith and not by works such as:
Romans 3:23-26, “For all have sinned and fall short of the glory of God, and are justified by his grace as a gift, through the redemption that is in Christ Jesus, whom God put forward as a propitiation by his blood, to be received by faith. This was to show God’s righteousness at the present time, so that he might be just and the justifier of the one who has faith in Jesus.”
Galatians 3:10-14, "For all who rely on works of the law are under a curse; for it is written, "Cursed be everyone who does not abide by all things written in the Book of the Law, and do them. Now it is evident that no one is justified before God by he law, for “The righteous shall live by faith.” But the law is not of faith, rather “The one who does them shall live by them. Christ redeemed us from the curse of the law by becoming a curse for us-for it is written, “Cursed is everyone who is hanged on a tree” – so that in Christ Jesus the blessing of Abraham might come to the Gentiles, so that we might receive the promised Spirit through faith.”
You can cite verses about Christ being the only way to challenge their view, such as John 14:6, Acts 4:12, and 1 Timothy 2:5 as well. I hope this helps!