Because God wills to manifest Himself in 3 ways, or 10, or 1000, or ONE, doesn’t make Him that many persons. “Hear, O Israel, the Lord your God is ONE Lord.”
He can also manifest Himself in many presentations simultaneously, but He remains One Lord. Who spoke to and by Isaiah declaring there was only One Lord and One Savior, and no other?
Isaiah 43:11 (KJV)
I, even I, AM the Lord ; and beside Me there is no Saviour.
Isaiah 59:16 (MKJV)
And He saw that there was no man, and wondered that there was no intercessor. Therefore His own arm brought salvation to Him; and His righteousness sustained Him.
Was it the Father speaking to him or the Son? Which?
I’ll tell you something else. If I pray to God, and HE answers me, who is it that answers? They or He? When in the early seventies, I cried out to the Only Name under Heaven whereby I could be and was saved, Jesus Christ, who answered me? Jesus Christ did.
And with Thomas, I cried out, “My Lord and my God,” and He not only did not correct or rebuke me, but He also SAVED ME. Jesus Christ, the One Who spoke to Isaiah saved me, He and He Alone, the One God of Israel and of the universe.