The most important metaphysical assumption is the loving creator.
This is really The Key to understanding reality - and it is, indeed, the key to the possibility of understanding anything.
This key was given in the person and teachings of Jesus; but it is also directly-available to anybody and everybody by personal revelation of the Holy Ghost.
It is because God is creator that God knows; and it is because God loves us that he has made this a world in which we too can know.
And - assuming the truth of the loving creator - without knowledge of the loving creator, we will necessarily misinterpret, will fail to understand, everything in this world; because we will be attempting to understand reality in a false context: a context lacking a loving creator makes the world not-necessarily coherent, thus unknowable; and our own 'knowledge' (of any-thing, even of context) arbitrary or self-contradictory.
This is ultimately why Christianity was conducive to theology, philosophy and science... because the assumptions of Christianity make rational the hope that Man can Know.
Ultimately, the only reason to believe in the loving creator is that it is true; and that it is true can only be known by personal revelation, and by the further recognition that personal revelation is the ultimate form of knowledge. So this must also be assumed.
Once the loving creator has been believed, once known - had-faith-in; then (in principle) everything potentially makes-sense to us.
Conversely, lacking belief in the loving creator, there is no reason why we would be able to know anything - and indeed this is the implicit (sometimes explicit) position of other religions and of atheism: i.e. that Man does not understand anything.
In sum: more than just the reality of a creator deity is necessary for human knowledge - because real human knowledge also requires the loving 'attitude' of God (the creator) towards Men.
Thus a personal creator God, God-as-a-person (capable of love), is a necessary requirement for there to be any possibility of human knowledge.