What fellowship? The Apostles wree not fwllos with Jesus. They called Christ their Lord (kyrios); He was their teacher and master, and they were his disciples, students.
In the NT the Apostles refer to themselves as servants, not fellows of Christ. Fellowships are for fellows, something that is between equals.
There is nothing in the term koinonia that implies any equality of members. All members of the American Medical Associations are fellows; all American citizens are fellow citizens with equal rights; military officers are fellow members of the Officers' Mess, etc. But we are not fellows with God no matter who saintly we may think we are.
We may be in communion with other members of the same faith, professing one and the same God, and we may think of ourselves as a community (fellowship) of believers, the Church, and some may even think as being "in" Christ, but there will always be God whose ways and thoughts are not ours and we can never be what He is by nature, even though we may be with Him by grace. It will always be, the Lord-servant servant relationship, glorfying God, and not a fellowship.
Insisting that one can read the Bible only if he has the "fellowship" and is privy to special "gifts," i.e. on the "membership" of special knowledge and perception, is pure Gnosticism. Needless to say, Gnostics found Paul very close and dear to their hearts, for obvious reasons. And Gnosticism, to my surprise, seems to be the backbone of Portestanitsm.
2 Cor 13:14, "The grace of our Lord Jesus Christ and the love of God and the fellowship of the Holy Spirit be with all of you."
1 Cor 1:9, "God the Father, through whom you were called into fellowship with His Son, Jesus Christ our Lord."
Phil 2:1-2, "Furthermore if there is any encouragement in Christ [and there is], if there is any comfort from love, and if there is any fellowship with the Spirit; if there is any affection and mercies, bring to completion my happiness that you might be thinking the same things, having the same virtue love, united in soul, intent on one objective."
The Christian way of life emphasizes our remaining in fellowship with Him. When a believer sins, they fall out of fellowship. This is not the same as the sealing or indwelling of the Holy Spirit, but is associated with phrases such as 'walking in the Spirit', or fellowship with Him.
We only advance in the Christian walk while we are in fellowship with Him. We advance by Him further sanctifying us in faith. All true faith comes from Him. The more faith and doctrine He inculcates in us, the more He sanctifies us.
This also explains how so many believers may slide out of fellowship, attempt to study the Bibel on their own out of fellowship and simply harden their hearts, rather than allowing Him to sanctify them in their spirit, their minds, their hearts, and even physiological impacts in their bodies.