People who reject that a Christ isn’t part of the trinity, are liars. John wants us to keep these questions in our minds to keep our defenses up and keep us vigilant. There’s a lot of religious groups that accept God the father or Jesus or the Holy Spirit, but not all three persons as one triune God. So anyone who thinks or accepts that Jesus is less than eternal God, fully human and divine, is a liar or is a false religion and fits the description of false teaching. So fellow believers look for the truth, look at Gods word. Study and meditate in it, and don’t be deceived.