Excerpts from Berkhof's "Systematic Theology" on the atonement

According to Scripture the moving cause of the atonement is found in the good pleasure of God to save sinners by a substitutionary atonement.

... It was the love of God that provided a way of escape for lost sinners, John 3:16. And it was the justice of God which required that this way should be of such a nature as to meet the demands of the law, in order that God "might be just, and the justifier of him which believeth in Jesus," Rom. 3:26.

... The fact that God gave up His only begotten Son to bitter sufferings and to a shameful death cannot be explained on the principle of His love only.

... Reformed theology in general rightly shows a decided preference for this view [that the atonement was absolutely necessary, rather than relatively or hypothetically necessary or even unnecessary].Whatever may be true of Beza in later life, it is certain that such scholars as Voetius, Mastricht, Turretin, a Marck, and Owen, all maintain the absolute necessity of the atonement and ground it particularly in the justice of God, that moral perfection by which He necessarily maintains His holiness over against sin and the sinner and inflicts due punishment on transgressors.

(Louis Berkhof, Systematic Theology, Banner of Truth, 2003, pp. 367-369)