In the Gospel of John, the personal nature of the act of faith is stressed by the very use of the verb “to believe.” In the Gospel, we encounter the expression “to believe,” which means to lend credence to or hold to be true. For instance, to believe Scripture (Jn 2:22), or Moses, or Christ (Jn 5:46). We also encounter the expression “to believe that,” meaning to be convinced that, or just to believe. For instance, to believe that Jesus is the Holy One of God, that he is the Christ, that the Father has sent him.
But alongside these well-known usages, there is one unknown to profane language yet most dear to the Evangelist, and that is the expression “to believe in,” as in the sentence: Let not your hearts be troubled. Believe in God, believe also in me (Jn 14:1). Believe here means: have faith in, entrust yourself to the person you believe in, build your own life on that person. It indicates a total and unconditional trust that is to replace all human insecurity. A trust in consequence of which the heart can never again be troubled by anything. Jesus asks the same kind of trust for himself that God asked of his people in the Old Testament.
Believing in the Son of God is something different and more than believing that Jesus is the Son of God…. As regards the former, there are all sorts of degrees and you would never finish progressing through them. In other words, you can always trust more in Christ, by surrendering yourself to him more and more and losing yourself in him, until faith in the Son of God becomes the whole reason for your life. Like Paul, who could say: The life I now live in the flesh, I live by faith in the Son of God who loved me and gave himself for me (Gal 2:20).