"truly our fellowship is with the Father
and with His Son Jesus Christ"
After Peter preached on the Day of Pentecost (Acts 2) many believed and over three thousand were saved that day.
The apostles began presenting the things about Christ and the New Covenant Kingdom and, through the Spirit, the new Believers received powerful understandings and experiences.
And they -
"... continued steadfastly in the apostles’ doctrine and fellowship, in the breaking of bread and in prayers," Acts 2'42.
Some of them would have heard Jesus speak about "Our Father" on the mountain (Matthew 6'9-13).
Now they could experience the immediate intimacy of the fatherhood of God, calling Him Father.
That's fellowship. And it lies at the heart of the life and worship of God's People.
Galatians 4'6 says: "God has sent forth the Spirit of His Son into your hearts, crying out, 'Abba, Father!',” .
And 1 Corinthians 1'9: "God is faithful, by whom you were called into the fellowship of His Son, Jesus Christ our Lord".