God is not seen an isolated individual but as a loving, interpersonal communion to which we owe our very existance. It is not a leap in the dark to believe this. Historically it is grounded in Jesus' resurection. God manifests His triune nature by raising Jesus from the dead by the Spirit. In the bodily resurection of the Son through the Spirit, trinity is demonstrated and grounded.
Unity can admit to great complexity.
The human body, for example, is very complex organisms working together in unity. God loves sinners in history because, before that, Father loves the Son and Spirit, and then loves us in relation to them. The love God has for sinners flows from the love that flows everlastingly within the trinity. God's love for us sinners is not just the love of a single, unitarian subject. John here makes known the Triune love that flows among the Persons of the Godhead.