“…that which we have seen and heard we declare to you, that you also may have fellowship with us; and truly our fellowship is with the Father and with His Son Jesus Christ.”
Genuine fellowship has a substructure, a spiritual platform. That substructure is not our personalities or capacity for friendship but something more substantial than that. Our horizontal fellowship is possible because of our vertical fellowship with God.
and truly our fellowship is with the Father
The main purpose of the epistle of 1 John is to show how we can fellowship with God.
Fellowship with the Father is the second kind of fellowship in this verse. It is the vertical fellowship. Horizontal fellowship among Christians is not possible without a prior vertical fellowship with the Father and the Son.
and with His Son Jesus Christ
Jesus alone makes fellowship with the Father possible. God reconciled us to Himself in Christ. Without Christ, our only identity with the Father is a Creator/creature relationship. With Christ, we can relate to God as a member of His family. Jesus made it possible to fellowship with the Father. We can fellowship with both the Father and the Son.
Vertical fellowship is the substructure of all fellowship.
Eternal security and eternal assurance are two different ideas. Once we receive Christ as our Savior, our salvation is never in jeopardy (Jn 4:14; 6:32, 37-40). However, false teachers always come along, trying to cast doubt on our salvation. God wants us to know how we can fellowship with Him with assurance. We can have the assurance of fellowship with God based on the blood of Christ (1 Jn 1:7).
Fellowship is impossible without a relationship. God provides the means of that relationship in His Son.
1 Jn 1:7, “But if we walk in the light as He is in the light, we have fellowship with one another, and the blood of Jesus Christ His Son cleanses us from all sin.”
Probably no one will ever introduce you to the President of the United States. It would be nice to meet him, but very few do. It is one thing to know about the President, but it is another to meet him personally. As well, it is one thing to hear about the Lord Jesus, but it is another thing to meet Him personally in salvation.
Many Christians know facts about the Father and the Son, but they do not fellowship with them. This is the difference between religion and Christianity. Religion talks about God but Christianity fellowships with Him.