Obviously, it is a personal construction, see Smyth § 1982. Ho Logos is primarily the subject of amfibeble^tai, thus in the nominative, and also, secondarily, of the infinitive.
Now the meaning is not so clear for me. I understand :
"For, if one does not doubt that the unique Reason of God is the essence of virtue in everyone..."
Can you give the true meaning ? Is it a sentence by Plotinus ? Can you give some reference ? Or the rest of the sentence ?