God is love
I will quote directly the words of Pope Benedict XVI in his homily last 2011 regarding the Most Holy Trinity: "When one thinks of the Trinity, what really comes to mind is the mystery of God who is Three in One, one God in three Persons. Instead, today's liturgy draws our attention to the reality of love that is contained in this first and greatest mystery of our faith. The Father, Son and Holy Spirit are one, because God is love..."
How wise is the Holy Father to focus on love as the primary aspect of God's Unity. "It is the Father who has given everything to the Son; the Son receiving everything from the Father with gratitude; and the Holy Spirit as the fruit of this mutual love between the Father and Son."
The first reading reflects the love of God the Father, "The Lord is God indeed, there is no other." Keep the commandments and you and your children shall prosper. His commands are all based on love.
Jesus in the gospel for today also echoes the call of the Father to follow the commandments and much more. "Know that I am with until the end of time." That is love.
The Holy Spirit is the fruit of the love between the Father and the Son. In the second reading, "The Spirit himself and our spirit bear united witness that we are children of God."
It is love that makes us one with the Father, Son, and Holy Spirit. Grant that we may also be one with one another simply because God has loved us first.