The Old Testament Israelites had the Scriptures, and the Scriptures tell of God the Son and God the Spirit.
Moses spoke of the Lord Jesus ( Deuteronomy 16:15-22 ), and all Israelites knew of the Holy Spirit ( 1 Samuel 10 and many other places attest to this ) .
David also spoke of His Son in the Psalms, and His Spirit as well.
In Jesus' time, the Pharisees asked Him point blank if He were the Son of God:
" But he held his peace, and answered nothing. Again the high priest asked him, and said unto him, Art thou the Christ, the Son of the Blessed?
62 And Jesus said, I am: and ye shall see the Son of man sitting on the right hand of power, and coming in the clouds of heaven."( Mark 14:61-62 ).
...So the Jews already knew of Him and who He is, from their own Scriptures.
Additionally, all men know of the Godhead, and we are without excuse;
God has revealed this to us as part of His creation:
" For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who hold the truth in unrighteousness;
19 because that which may be known of God is manifest in them; for God hath shewed [it] unto them.
20 For the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, [even] his eternal power and Godhead; so that they are without excuse: " ( Romans 1:18-20 ).
May God bless you.