A triune relationship
For Christians, God is not an impersonal essence or a mere higher power but rather a person God with whom one may enter into an intimate relationship. Yet precisely because God is Eternal in his Oneness lies His source of uniqueness and diversity. When speaking of God one is always limited but human language with does not fully grasp Divine reality.
Like Jew and Muslims, Christians believe in one God, all three profess their faith in the God of Abraham, the God of Isaac, and the God of Jacob. But for Christians, God is One in Three Persons. God's revelation that God is three persons does not contradict God's oneness.
Christians are baptised in the Name of the Father and the Son and the Holy Spirit. Each of the Divine persons relates in a special and in a unique way with humanity, yet in their unity of Love it is always the One God who relates with human kind. Each person of the Trinity is uncreated, limitless, eternal, and omnipotent. Although one God, the Father, Son, and Holy Spirit are distinct from each other. For the Father is neither made nor begotten; the Son is not made but is begotten from the Father; the Holy Spirit is neither made nor begotten but proceeds from the Father and the Son (see the Athanasian Creed).
The Mystery of the Trinity is the main mystery of faith and Christian life. It is the mystery of God in Himself. It is the beginning of all the other mysteries of the Faith. It is the light that enlightens all other mysteries (Catechism of the Catholic Church, 232-261).
Catholic Doctrine on the Holy Trinity