God the Father and God the Son behold the infinite lovableness of each other. There flows between these two divine Persons a Love which is divine and personal. It is a Love so perfect that it is a living Love. This Love is God, the Holy Spirit, the Third Person of the Blessed Trinity.
The Father and the Son eternally give this personal Love (God the Holy Spirit) to one another. God’s love for us has led Him to make us sharers in His own divine life. We can say that the Spirit of Love, God the Holy Spirit, is the One Who makes this possible. Since the work of making souls holy is especially a work of divine Love, we say that the Holy Spirit is the Sanctifier, the One Who makes us holy; yet what one Person in the Trinity does, all three do.