For the Father. Faith, love, obedience, etc., are peculiarly and distinctly yielded by the saints unto him; and he is peculiarly manifested in those ways as acting peculiarly toward them: which should draw them forth and stir them up thereunto. He gives testimony unto, and bears witness of, his Son: “This is the witness of God which he has testified of his Son” (1 John 5:9). In his bearing witness he is an object of belief. When he gives testimony (which he does as the Father, because he does it of the Son) he is to be received in it by faith. And this is affirmed, “He that believes on the Son of God, has the witness in himself” (1 John 5:10). To believe on the Son of God in this place is to receive the Lord Christ as the Son, the Son given unto us, for all the ends of the Father’s love, upon the credit of the Father’s testimony; and, therefore, therein is faith immediately acted on the Father. So it follows in the next words, “he that believes not God” (that is, the Father, who bears witness to the Son) “has made him a liar” [1 John 5:10]. “You believe in God,” says our Savior (John 14:1); that is, the Father as such, for he adds, “Believe also in me”; or, “Believe you in God; believe also in me.” God, as the prima Veritas upon whose authority is founded and whereunto all divine faith is ultimately resolved, is not to be considered hupostatikōs, as peculiarly expressive of any person, but ousiōdōs, comprehending the whole Deity; which undividedly is the prime object thereof. But in this particular it is the testimony and authority of the Father (as such) therein, of which we speak, and whereupon faith is distinctly fixed on him—which, if it were not so, the Son could not add, “Believe also in me.”
Communion with the Triune God (Wheaton, IL; Crossway; 2007) p. 98.