The salvation proclaimed in the gospel is not some mechanical operation that God took on as a side project. It is a ‘mystery that was kept secret for long ages’ (Rom. 16:25), a mystery of salvation that goes back into the heart of God, decreed ‘before the foundation of the world’ (Eph. 1:4; 1 Pet. 1:20).
When God undertook our salvation, he did it in a way that put divine resources into play, resources which involve him personally in the task.… The deeper we dig into the gospel, the deeper we go into the mystery of the Trinity."
— Fred Sanders, The Deep Things of God: How the Trinity Changes Everything, (Wheaton, IL: Crossway, 2010), page 13