The lie that a believer can go in and out of fellowship is being taught at the university level on down to the church level. It is not scriptural to have such a belief. It is a lie from the enemy. Here is why.
Fellowship refers to oneness, a unity with one another. With this concept of fellowship applied to a believer in Christ, it is impossible to be separated from someone who lives in you.
The author of Hebrews writes in Hebrews 13:5, "Make sure that your character is free from the love of money, being content with what you have; for He Himself has said, "I will never desert you, nor will I ever forsake you,"
In 1 John 1:7, John writes, "But if we walk in the Light as He Himself is in the Light, we have fellowship with one another, and the blood of Jesus His Son cleanses us from all sin." If you practice the truth by realizing that you are walking in the light of His grace at every moment, you will know by first-hand experience what it means to live in fellowship together with your Father and what it means to continuously be kept completely cleansed from all sin by the blood of Jesus.
You are in fellowship with God all the time. Your perception of that may change (like it did for the prodigal son), but remember: your feelings are not the
standard of truth. The promises of God in Scripture are the truth. When we understand the truth, it causes us to want to live a lifestyle that glorifies Him. Our "wanter" is changed as we are transformed by the renewal of the mind as a new creation. Amen! :-)