Therefore, as for you, let that remain in you which you heard from the beginning. If that which you heard from the beginning remains in you, you also will remain in the Son, and in the Father.
In the Father, and in the Son ... This is a reference to the corporate body of Christ, a conception that was announced by the Lord himself in the analogy of the true vine (John 15:1-10).  Thus John takes his place alongside Paul in the presentation of salvation "in Christ."  A heavy emphasis upon this has been given throughout this whole series of commentaries; and, for a fuller discussion of it, see  Romans 3 in my Commentary on Romans.  Significantly, John here made adherence to the original gospel a prerequisite of abiding in God and in Christ.  As Stott noted: