Charles Ellicott Commentary John 15:4

"Abide in me, and I in you. As the branch cannot bear fruit of itself, except it abide in the vine; so neither can ye, except ye abide in me." — John 15:4 (ASV)

Abide in me, and I in you.—The clauses are here connected as cause and effect. The second is the promise, which will not fail if the command of the first is observed. The union then, and all that follows from it, is placed within the power of the human will. All is contained in the words, “Abide in Me.” The one who obeys this command has Christ abiding in him, and is a fruitful branch of the true vine.

As the branch cannot bear fruit of itself.—The branch regarded by itself, apart from (“except it abide in”) the vine, has no original source of life. The sap flows from the vine to branch and tendril and leaf and fruit. The branch by itself is a lifeless organ, and only fulfills its functions when it is connected with the vine.

So in the spiritual life, people apart from Christ have no original source of life and fruitfulness. The true life flows from Him to every branch that abides in Him, quickening by its power the whole person, and making them fruitful in good. The person who lives without faith in God may be said to exist, rather than to live, and misses the true aim of their being.