True believers personally know the only true God because He has made His home in them. They are his children, born of Him their Abba Father. They know the only true God personally. The children of God know their Abba Father.
“Believe in God, believe also in me. In my Fathers household are many ABODES... I am the way... no one comes to the Father but through me..... If you had known me, you would have KNOWN my Father also; from now on you KNOW Him, and have seen Him.... If anyone loves me, he will keep my word and my Father will love him, and We will come to him and make Our ABODE with him.“ John 14
“Father.... THIS is eternal life, that they may KNOW You, the only true God, and Jesus Christ whom You have sent.“ John 17:3