Father, Father, Father
The Greek word pater is strewn throughout the Bible and used in different contexts. In English we distinguish a biological father from a stepfather or a foster-father. In Biblical Greek they're all pater.
Your biological father had a father, and his father was your grandfather. You guessed it. They're all just pater to you. You know the difference between a teacher and a boss. In Bible times you might have addressed both as pater. In fact, every male ancestor in your family tree, every man who had jurisdiction over you, and every man who influenced you by mentoring or instructing you, might have been considered pater to you if you spoke Greek and lived in New Testament times.
The key to understanding pater in the Bible is in looking at the context surrounding its use, and attaching any articles that should apply. My father is different than his father. A nation's father is different from a father of a particular clan. The father of a company could be the company founder or a present manager or department head.
But there is only one The Father and Jesus clearly stated that The Father was His Father. In the New Testament, Jesus further stated that by means of adoption His Father, The Father, could become Our Father, too. When you genuinely recognize God as your source, submit to His sovereignty, and follow His instruction and guidance you actually take God as your father and become a brother of Christ. Biblically speaking, you follow after Christ to join the family of God. Interesting, huh?