There is an unfortunate tendency when it comes to interpreting John 3:16. John says, “whoever believes in him” will not perish. Some people think that “believing in” Jesus means nothing more than giving assent. They hang their hopes for heaven on this slender reed: “Sure, I believe in Jesus.” They mean that they believe that He exists and agree with at least some of what the Bible says about Him. But the biblical teaching about Jesus means practically nothing to them. They “believe in” Jesus in much the same way a child “believes in” Santa Claus. It is the particular legend or story with which they were brought up.
But John means much more than this when he writes that whoever “believes in” Jesus Christ will not perish. In his outstanding study of John’s Gospel, C. H. Dodd points out that this Greek construction translates a common Hebrew phrase in the Old Testament that employs a form of amen, a word that signifies something that is firmly held or established. Isaiah used it in his famous statement to King Ahaz: “If you are not firm in faith, you will not be firm at all” (Isa. 7:9b). Therefore, when John says “whoever believes in him,” he is speaking of those who give their “Amen” to Jesus, embracing Him as a trustworthy Savior and committing themselves to Him.
Richard D. Phillips, Jesus the Evangelist (Kindle Edition, location 1108)