Here is what the Lord Jesus says about the way that one becomes a child of God:
You have heard that it was said, ‘You shall love your neighbor and hate your enemy.’ But I say to you, Love your enemies and pray for those who persecute you, so that you may be sons of your Father who is in heaven; for he makes his sun rise on the evil and on the good, and sends rain on the just and on the unjust. (Matthew 5:43-45, RSV2CE; emphasis added)
The phrase so that is an adverbial particle that expresses purpose in Greek, as is indicated by the English above: in order to be a son of God you must love your enemies and pray for your persecutors. He does not say “so that you can remain a son of God” nor “so that you can show you are a son of God, but rather so that you may be a son of God.
Obviously this passage does not play well with the Protestant idea of sola fide. There is nothing about faith in this passage, and certainly nothing about faith alone. As we have said before, this doesn’t mean that faith is irrelevant: that would be an argument from silence. But we can say that our works are associated with our justification before God in some way, because that is just what is said in the passage above. It does not seem that one can reasonably say that Christ is talking about something that happens after justification, because the language of the passage refers to purpose and not to result.