There are a few occasions where God, in his word, the Bible, identifies himself as “one”. Deut. 6:4, for example, says:
Hear, O Israel: The LORD our God, the LORD is one.
This is quoted in Mark 12:29.
Elsewhere in the Bible, we are told that this one God is the Father:
…there is one God, the Father… (1Cor. 8:6)
(Also see John 17:1-3.)
The importance of knowing who the one, true God is is highlighted in Deut. 6:4-6 (and Mark 12:29-30):
{4} “Hear, O Israel: The LORD our God, the LORD is one. {5} You shall love the LORD your God with all your heart and with all your soul and with all your might.
We are to recognise who the only God is so that we can love him entirely, with all of our being.
Hear, O Israel: The LORD our God, the LORD is one.
One of the best references on this topic is 1 Tim.2:5.
“For there is one God, and one mediator between God and men, the man Christ Jesus.”