“Do not let your heart be troubled; believe in God, believe also in Me.” John 14:1 NASB
This brief statement “Believe in God, believe also in Me” shows that the Son has real existence distinct from the Father, and is not merely a mode or name of the same individual being. And one must note that He does not say “Believe in the Father, believe also in Me’, so that one might argue that He is here teaching to believe not only in one mode of the Supreme Being, but two modes, but rather, by not merely distinguishing Himself from the Father, but from “God”, He shows that He is not Himself the God He is distinct from, but another distinct individual being besides Him. And certainly, if one will inquire into which ‘God’ the Lord intended to signify here by the word ‘God’, and enjoined His disciples to trust in, it must be acknowledged to be none other than the God of Israel, the God of Abraham, Isaac, and Jacob; the one and only true God, the Supreme Being, Who alone is the supreme Cause of all things, and supreme Ruler over all things. It is this God that Christ distinguished Himself from.
Now the modalists, including so many who falsely style themselves as trinitarians when they are not, cannot consent to such language as our Lord uses here. For to believe in God, according to them, requires one to believe in the Son and Spirit as well, for they define God as being Father, Son, and Spirit all together. And for this reason, they rage against anyone, for instance, who says that Jews, or Muslims, worship the same God as Christians; for they insist that such is impossible, when the God of Christians is ‘triune’ or ‘tripersonal’, while the God Muslims and Jews try to worship is one person only. And so according to their standards, no one can worship, or believe in, the true God, the Almighty, the God of the Bible, without worshipping and believing in Father, Son, and Holy Spirit together; and to believe in God, for them, is to, in the same exercise of faith, believe in the Son, as well. And this is a necessary thing for them to think, who make the Father and Son out to be one and the same individual being.
Yet the Son of God here refutes them with only a few words when he says “Believe in God, believe also in Me”. For He makes clear, by His statement, and especially by the term “also”, that to believe in God and to believe in Him are two distinct things; and so it follows, according to the words of Christ, that one could believe in God, without believing in His Son. Now this shows that God and His Son are two distinct individual beings, or else, it would truly be impossible, as the modalists say, to believe in God without believing in Christ, Christ being the very self-same individual being. It necessary follows, from the words of the Lord here, then, that the Son is a distinct individual being from the Father, not the one God Himself.