Before we make our own this first exclamation of the Lord's Prayer, we must humbly cleanse our hearts of certain false images drawn "from this world." Humility makes us recognize that "no one knows the Son except the Father, and no one knows the Father except the Son and anyone to whom the Son chooses to reveal him," that is, "to little children." The purification of our hearts has to do with paternal or maternal images, stemming from our personal and cultural history, and influencing our relationship with God. God our Father transcends the categories of the created world. To impose our own ideas in this area "upon him" would be to fabricate idols to adore or pull down. To pray to the Father is to enter into his mystery as he is and as the Son has revealed him to us.
The expression God the Father had never been revealed to anyone. When Moses himself asked God who he was, he heard another name. The Father's name has been revealed to us in the Son, for the name "Son" implies the new name "Father."
Source: Catechism of the Catholic Church (Vatican.va)