Then Jerusalem and all Judea and all the region about the Jordan were going out to him, and they were baptized by him in the river Jordan, confessing their sins. But when he saw many of the Pharisees and Sadducees coming to his baptism, he said to them, “You brood of vipers! Who warned you to flee from the wrath to come? Bear fruit in keeping with repentance. And do not presume to say to yourselves, ‘We have Abraham as our father,’ for I tell you, God is able from these stones to raise up children for Abraham. Even now the axe is laid to the root of the trees. Every tree therefore that does not bear good fruit is cut down and thrown into the fire.
Matthew 3:5–10 ESV
So writes Matthew about the message and actions of John the Baptizer. Notice his purpose in speaking of a tree and it’s fruit. John wants people to repent of their sins and engage in new obedience. Claiming to be Abraham’s descendants is not enough. They have to, to borrow the language of the Apostle Paul, “also walk in the footsteps of the faith that our father Abraham had” (Romans 4:12).
So the point of the tree analogy is to get his hearers to act. To interpret the message in a way that makes you not realize you should act, but rather wonder what kind of “heart” you have, is a misuse of Scripture. When God calls for people to do things, we should not substitute action with speculation about what kind of persons we are.
So while Augustine was right and Pelagius wrong in their grand views of grace, beware confirmation bias. Not every passage in the Bible is commentary on Augustine’s doctrines. And, while I am open to specific arguments otherwise, I think the vast majority or all of the fruit analogies in the Bible are aimed at telling people not the rely on their nominal status (“Israelite,” “Christian,” “elect,” etc.), but instead to follow God’s commands and turn away from sin.