This unusual meeting between Abraham and three men is actually a divine encounter in which God takes on human form. On rare occasions in the Old Testament, God appears in some physical way to get his point across. (This kind of communication culminated 2,000 years later when Jesus of Nazareth was born; one of his titles was Emmanuel, which means “God with us.”)
Perhaps you’re thinking that you’d have no problem believing in God if he were to appear to you miraculously. But neither Abraham nor Sarah viewed this encounter as a summit meeting with God. As a matter of fact, Sarah was quick with a doubter’s laugh (as Abraham had been; see Genesis 17:17) and even lied right to God’s face.
Be cautious of the unusual or bizarre in the spiritual realm. Faith that is built primarily on such infrequent happenings doesn’t have a solid foundation. God can and sometimes will do unexpected things that will affect you personally. As you seek out God’s direction in your life, you’ll discover him at work in the everyday occurrences. And it’s the perception of God’s hand in everything you do that provides the firmest foundation for building your faith.
Taken from NIV The NIV Journey Bible