@Eddi
Abraham is most likely the 'Heavenly father' referred to by Jesus in gospel quotations. Abraham is the father of many nations, whose children are like the stars in the sky. (See Genesis 15:5) Abraham is the one who puts away war. He refuses to kidnap people. He abolishes idolatry. He is the father of all who believe in peace. Cain is the father of all murderers. All murders were in Cain, the first murderer. All peaceful people are children of Abraham. When they die they go to Abraham's bosom, meaning they are counted as his.
In most scripture the concept of a father is very important. A father is a highly respected and revered person who contained you before you were born. All of his potential descendants are in him. A reference in Hebrews refers to this when it says Levi's descendants paid a tithe to Melchizedek through the tithe paid by Abraham. There are other such references, too. Abraham being the father of many nations who are as the stars in heaven: he is the heavenly father.
I'm not saying Abraham is 'God'. That is an entirely different thing. Both God and the Trinity are philosophically derived concepts. Abraham might be referred to as a theos occasionally in scripture, depending upon what is being said. Other people are, too, such as all of Israel's judges. It is not always reserved for the same thing, so the translation of theos may be confusing when reading an English verse.
All arguments by Paul for Christianity aim at claiming that Christians are children of Abraham. If he proves we are children of Abraham then he considers his case made. Consider Abraham and the Bosom of Abraham, both of which appear in the gospels directly. These are 'Heavenly father' 'Abrahams bosom'. There is no other mention of a heavenly father in the old scriptures. The LORD never calls himself Israel's father, except in a sense of adoption. "Today I have become your father," and that only happens in Psalm 2. So if we also are adopted, then what is the difference? That is Paul's theme.
Have you ever heard protestants speak of the of Catholics praying to Saints? What about when Jesus prays to Abraham in Matthew 6:9 ? How is that any different? I see that it is the same thing.
All of the arguing we have about whether Jesus is a person or not comes from general ignorance among all people. There is no need to argue. Almost nobody anywhere knows what's really going on in my opinion.