Not grasping a concept, not having something yet revealed to you, and denying God's revelation are different things. Moreover I do not agree that all did not grasp it then. Concerning Genesis 18:1-3 St. Augustine says: "Do you see that Abraham Three but bows down to One... Having beheld Three, he understood the mystery of the Trinity, and having bowed down to One, he confessed One God in Three Persons."
Moreover, Jesus explicitly taught this to His disciples and expected them to hold it, which they did in fact do. Of His own teachings He thanks the Father saying: "I praise you, Father, Lord of heaven and earth, that you have hidden these things from the wise and intelligent and have revealed them to young children. Yes, Father, for this was pleasing before you." The disciples being the "little ones" here. Man of his own power can not grasp this but God can reveal it to man, and did. To those who He has "opened the mind to understand" it is quite simple, which is why the Apostolic Faith is even given to and taught to children, it is not beyond them.
Believing into Jesus is certainly necessary and the requirement of salvation but when you fundamentally deny who a person is how can you be said to believe in them? If I tell you I am a human and you tell me I am a dog (like as Jesus tells the world He is truly God and eternal and the non-Trinitarians say He's just a creature made from nothing just like they are, which is an INFINITE denial of who He is: for the gap between God and creation is infinite, the mind can not even fathom the distance between us and God) how can you say that you trust in me or believe into me or even know me?
There is only one possible way, and this is how some non-Trinitarians (and also pagans, fleshly Jews, etc) have been saved: honest ignorance which is not a willful ignorance.
But notice what this does: you can not say that this person who is honestly ignorant of Abaxvahl's humanity is a believer (or confessor) of my true humanity and me not being a dog. It puts them outside of the realm of those who believe that, just like non-Trinitarians, the subset of them who are honestly in ignorance.
This does not mean, as I just said, they can not be saved, but it does mean that they can not be truly said to be in the camp of those who confess what Jesus said He was, or as I say "doctrinally Christian." Someone who is doctrinally Christian, that is holds and believes in the Christian teaching (which is nothing other than what Christ taught), can be called that. Someone who doesn't can not be called that it would be a lie, that is why I say they are not Christian. They deny either out of malice or an honest ignorance what Jesus revealed about Himself, meaning they do not even really know Jesus as they deny His fundamental identity which He spoke of Himself (that is being the Second Person of the Trinity, the natural Son of God).
Their salvation is in the Lord's hands, we can't judge souls, but we can judge positions, and their position is not doctrinally Christian, and so it may be rightly said that they are not Christian, just as someone who holds to Buddhism is not doctrinally Christian, or someone who holds to Islam's teachings is not doctrinally Christian, or an atheist isn't, even though it be possible any of these people can be saved.