We can count on God's love. This kind of love is catalytic to the Christian life. There is great comfort and security in being loved by someone who will not be daunted by anything in us. Since it is true that we have fellowship with the Holy Spirit. Paul assumes this as a fact.
Christians commonly participate in the same things as the Holy Spirit.
They may refer to the fellowship that comes from the Holy Spirit. If we have a community with the Holy Spirit then we are ready for community with other Christians. It is the Holy Spirit who will bring unity to a local church. He alone can bring cosmos out of chaos, order out of disorder. If the Holy Spirit joins in mutually with us, he will put our hearts right.