In the Old Covenant, belief in the One God was relational rather than merely rational. It wasn’t just a theoretical acknowledgment that there is only One Supreme Being. The daily repeated Shema says, “Hear, O Israel: The Lord is our God, the Lord alone. You shall love the Lord your God with all your heart, and with all your soul, and with all your might” (Deut 6:4 NRSV). To know the One God is to love him totally and to serve only him.
As a devout Jew, Saul of Tarsus loved and served only the God of Israel as the one who is in a class of his own. However, after the risen Jesus encountered Saul, (now known as Paul), he explicitly and continually spoke of his relationship with the risen Lord Jesus in the same exclusive way. And he urged other believers to turn from idolatry to love and serve the risen Jesus as the only true Lord.
We also come to know Jesus as the one true God as he encounters us bringing us into a relationship with him. That means grasping who Jesus is does not depend on our own ability to grasp the message concerning him. It rather depends on our being grasped by Jesus as he encounters us personally by the Spirit. And so, we do not come to know the One God as a mere object of intellectual knowledge, but only in Jesus as he moves towards us personally, dynamically, and relationally.