That's the point - a lot of people write/act as if "fruit" in Matthew 7:18 refers to deeds; good works.
I'm saying that I don't think it does.
Look at Matthew 7:16-17; Jesus says that false prophets may look like sheep, but they wear wolves' clothing and are dangerous - and we will recognise them by their fruit. Yet atheists, those who don't acknowledge God, leaders of cults and so on have still done good things - like founding charities, building hospitals etc. Yes, in God's eyes they are still sinners and still lost; but people could look at the outward "fruit" of charitable works and care for others, and think them to be Christians. Then, if for example they said they believed that the Trinity doesn't exist, like some cults do, people might be willing to listen/be deceived by them because their "fruit" is good - outwardly, they appear to be good people.
Yes, but unbelievers can do "true good" too. Like the Mormons, who have a family history tracing service and a Tabernacle choir, which no doubt sings and raises money for folks. Yet Mormonism is a cult.
Humanists are convinced that human beings are the highest power in the universe. They will often do good, for the sake of their fellow man (and woman).
Now if you're saying that because God made us all in his image, Genesis 1:26-27, and put his breath in us, Genesis 2:7, therefore in some small way God is in everyone; I think I'd agree with you. And if you are saying that it is God IN these people who is prompting them to think of others, show mercy, do good things etc; again, I think I would agree with you.
But it is not their good deeds that will save them. They are still sinners and need forgiveness - which they may never ask for because in their view, God doesn't exist and/or they want nothing to do with him.