About 3,000 years ago David wrote in Psalm 16:11: “…In Thy presence is fulness of joy…” He was right, for there is no greater joy than personal fellowship with God. Yet David could not know the fulness of joy now spoken of in Paul’s epistles, for he did not know Christ, who later came into the world as God, manifested in the flesh, to die for our sins. He did not know that Christ would make full satisfaction for sin and be raised from the dead to confirm our justification. Nor did David know that believers would be given Christ’s resurrection life, and a position, and “all spiritual blessings IN THE HEAVENLIES in Christ” (Eph.2:4-6; 1:3).
When Paul wrote that God has called believers “unto the fellowship of His Son”, he referred to a spiritual, heavenly fellowship, far more intimate and precious than any previously enjoyed by mortal man. This fellowship is to be enjoyed by faith, but it is faith based on fact, the fact that Christ indeed died our death and rose again from the dead that we might partake of His life and enjoy a position at God’s right hand in Christ. This is why the Apostle Paul urges believers of this dispensation of grace to “seek those things which are above, where Christ sitteth on the right hand of God” (Col.3:1).
Peter and John both came to know much of this fellowship through Paul, who was sent to Jerusalem “by revelation” to make known to the leaders there “THAT GOSPEL WHICH I PREACH AMONG THE GENTILES” (See Galatians 2:2-9; II Peter 3:15-18). This is why John writes in I John 1:3,4: “Truly our fellowship is with the Father, and with His Son, Jesus Christ. And these things write we unto you, THAT YOUR JOY MAY BE FULL”. source