John Calvin Commentary Luke 6:43

"For there is no good tree that bringeth forth corrupt fruit; nor again a corrupt tree that bringeth forth good fruit." — Luke 6:43 (ASV)

For the tree is not good—this statement, as related by Luke, appears to be a general instruction given by Christ: that by the fruits our opinion of every person ought to be formed, in the same manner as a tree is known by its fruit.

After having inserted the reproof to hypocrites, who “perceive a straw in the eye of another, but do not see a beam in their own,” (Luke 6:41–42), he immediately adds, For the tree is not good which bears rotten fruit, nor is the tree rotten which bears good fruit.

The illative particle γὰρ, for, appears to connect these two sentences. But as it is certain that Luke, in that sixth chapter, records various discourses of Christ, it is also possible that he may have briefly glanced at what Matthew explains more fully. I attach no great importance to the word for, which in other passages is often superfluous, and appears obviously to be so from the concluding statement.