Matthew 12:33

“Either make the tree good, and its fruit good, or make the tree corrupt, and its fruit corrupt, for the tree is known by its fruit.”

The alternative is then put in another way. It is a choice between making the tree good and or making it corrupt. As agriculturalists they would know that this was dependent on how it was tended and looked after, and whether it was in the hands of the right gardener. By responding to Jesus and His words they can ‘make the tree good', for He is the Master gardener. They can experience God's working in their hearts to ‘bless' them (Matthew 5:3). They can be ‘saved'. They can come under the Kingly Rule of God which has come upon them. They can become ‘trees of righteousness, the planting of the Lord' (Isaiah (Isaiah 61:3, contrast verse 13) The alternative is to turn away from God's mercy, and to neglect His forgiveness, and the offer of His Holy Spirit. Then the tree will become corrupt. It will go beyond the point of no return. And the result will be that it will produce corrupt fruit, fruit that is unwanted and unwelcome and inedible. And in the end, like any tree, each will be known by its fruit.

Later in Matthew 15:13 Jesus will refer to the Pharisees as plants which His heavenly Father had not planted, which would be rooted up. There too they were known by their fruits.