Anti-trinitarians would believe that the Bible since in a few texts would refer to God as "Echad," would imply that God, Jesus, and the Holy Spirit are not all actually God, but three separate beings. Where God the Father is not fully Jesus and fully God, but half Jesus and half God. Similarly for Jesus, Jesus isn't fully human or fully God, he's literally half human, half god, a demi-god, and the Holy Spirit is our concience. The biggest problem to overcome for anti-trinitarians is that shortly after the first versions of the old testament were written, the early Christians quickly made sure to clearify that "Echad" was what they believed. Historians know that the early Christians, who were mostly gentiles, wanted to show the Jews that God was "Echad" and eventually, the Jews believed. They also know that soon after Christianity begin to grow, the Jews changed the meaning of "one" in Deut 6:4, “Hear, O Israel! The Lord is our God, the Lord is one," which was written before the known Christ and Holy Spirit, and The Jews, after believing in Christians, removed "Yachid" from the text and replaced it with "Echad". This is where the Anti-trinitarian's argument begins to crumble.
Jesus in Matt 15:5 said, "For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh." This is important because Jesus used the word "hen" which is synonomis in the greek to "Echad," which implies that when Christ says "they will be one flesh," he's implying that they are united as one, which is what "hen" means. Jesus then quoted Deut 6:4 in Mark 12:29, and chose the word "hen" again when refering to God, "Hear, O Israel: The Lord our God, the Lord is one." That makes his word choice very important. He said that the Lord is "hen," that the Lord is "united as one." It shows that the Lord, united with Jesus and the Holy Spirit are all fully three in one. That is inevitably where the Anti-trinitarians arguements fall short.