"We cannot kept that which we do not have, neither can we have that which we cannot keep."
Writing to the assembly at Ephesus, the Apostle Paul said, "...being diligent to keep the unity of the Spirit in the bond of peace." That unity is the unity of the Spirit. It is the unity found in the Godhead between the Father and Son. It is the unbroken fellowship known and enjoyed between them - it is their peculiar relationship. That unity is also ours if we are "in Christ" - having been baptized into Him by the Spirit.
There can be no unity where this is not true. We cannot keep that which we do not have. Many seek and desire unity outside of the unity of the Spirit, because they "have not the Spirit". It is the Holy Spirit in me (in my spirit made one with His Spirit) that has fellowship with your spirit (if your spirit and God's Spirit are made one). For the Apostle said, "he that is joined unto the Lord is one spirit".
There is no unity outside of, or apart from, the unity of the Spirit. It is His unity, a unity He Himself brought us into. This is why ecumenicalism/ecumenism is a false unity and is not of God. The unity of the Spirit is fellowship with those who live in the Spirit. It is the fellowship of the Father and the Son, a fellowship that none can make and none can break.
"Truly our fellowship is with the Father and with His Son Jesus Christ." (I John 1:3). It is the Lord's longing to bring us all into that unity, that we may enjoy His fellowship - the "fellowship of the Spirit".