Turretin Answers Objections To Infant Baptism (7)

XVIII. The cause of infant baptism is not the actual faith of infants, of which they are no more capable than of that instruction by which the disciples of Christ are taught (Mt. 28:19). But it is both the universal command to baptize all the members of the church and the promise of the covenant made to parents and to their children equally (Acts 2:38, 39; Gen. 17*:7). The same thing must be said of the faith of covenanted infants as of their reason. Each is in them in the first act, not in the second; in the sowing, not in the harvest; in the root, not in the fruit; in the internal power of the Spirit, not in the external demonstration of work.

Francis Turretin, Institutes of Elenctic Theology, ed. James T. Dennison Jr., trans. George Musgrave Giger, vol. 3 (Phillipsburg, NJ: P&R Publishing, 1992–97), 19.20.18 (p.419).