The Holy Spirit - God's gift
He, then, gives us the "pledge" or "first fruits" of our inheritance: the very life of the Holy Trinity, which is to love as "God (has) loved us." This love (the "charity" of Cor 13) is the source of the new life in Christ, made possible because we have received "power" from the Holy Spirit.
Source: Catechism of the Catholic Church (Vatican.va)