How do Biblical Unitarians who accept Jesus’ sinlessness understand this argument?
1. All men are sinful.
2. If Jesus is not God, he is a man.
3. Thus, if Jesus is a man, he is sinful.
4. Jesus was not sinful.
5. Thus Jesus must be more than a man.
Let's assume, for the sake of understanding the final conclusion of this argument, that all of these premises/conclusions are in fact true. We could summarize them with a couple of additional conclusions/premises, as follows.
- Since Jesus is not a man, he is God.
- Because he is God, he is not sinful.
There is a hidden, subliminal message to this--one that Satan is keen on every person believing. It is this: only God can be sinless.
If this message is believed, then one cannot believe, or follow, Jesus' command given in Matthew 5:48.
Be ye therefore perfect, even as your Father which is in heaven is
perfect. (Matthew 5:48, KJV)
And if one cannot believe (trust) or follow (obey) Jesus' commandment, one does not love him.
If ye love me, keep my commandments. (John 14:15, KJV)
Therefore, to believe the five points listed in the OP is to oppose the teachings of Jesus, and to show one does not love him who gave his life for him or her.
Further, it sets up plain contradictions in the Bible that can be illustrated by the following list of premises.
- God claims to be immortal (see 1 Timothy 1:17; 6:16).
- Jesus, who is God, died (see 1 Thessalonians 4:14; Acts 13:30).
- God says He cannot be tempted with evil (see James 1:13).
- Jesus, who is God, was tempted (see Matthew 4:1).
- God claims He does not lie (see Numbers 23:19).
- Therefore, since God contradicts Himself, He cannot be trusted.
[Lies in italics]
Conclusion
The conclusions reached, if the original premises posted in the OP are followed to their full, show us that those premises cannot in fact be true, for they are self-contradictory.
As a non-trinitarian, I understand the argument presented in the question as invalid. It fails to properly summarize the Biblical teachings on the subject, and ends up teaching a message that is at odds with itself, with Jesus, and with his teachings.
Jesus saith unto her, Touch me not; for I am not yet ascended to my
Father: but go to my brethren, and say unto them, I ascend unto my
Father, and your Father; and to my God, and your God. (John 20:17,
KJV)