Trinity

The Trinity: God eternally exists as three persons, Father, Son and Holy Spirit, and each person is fully God, and there is one God.
Wayne Grudem

To know this God, who both condescends to share all that we are and makes us share in all that He is in Jesus Christ, is to be lifted up in His Spirit to share in God's own self-knowing and self-loving until we are enabled to apprehend Him in some real measure in Himself beyond anything that we are capable of in ourselves. It is to be lifted out of ourselves, as it were, into God, until we know Him and love Him and enjoy Him in His eternal Reality as Father, Son, and Holy Spirit in such a way that the Trinity enters into the fundamental fabric of our thinking of Him and constitutes the basic grammar of our worship and knowledge of the One God.
Thomas F. Torrance The Ground and Grammar of Theology pg. 155

Within the triune Godhead (Father, Son, and Holy Spirit), God has been uppermost in His own affections for all eternity. Therefore, God has been supremely happy in the fellowship of the trinity.
John Piper Desiring God pg. 33