3:15 should not perish. In comparison, if any lost sinner would merely look in faith to the Lord Jesus, “lifted up” (John 12:32) to die in judgment for his sins on the cross, he also will not only not die, but will receive eternal life. If “ye must be born again,” then “even so must the Son of man be lifted up.” Genuine saving faith, therefore, involves belief in the deity of Christ (John 3:13) and His substitutionary death (John 3:14). It will then result in the new birth (John 3:7-8) and everlasting life (John 3:15).
3:16 only begotten Son. Jesus calls Himself “Son of man” in John 3:13-14 and “Son of God” in John 3:16-18. As Son of man, He is the “heir” of God’s promises to man (Hebrews 1:2), man as God intended man to be, the perfect man. As Son of God, He is the unique, beloved, only begotten, eternally begotten, Son of the Father, “very God.”
3:17 sent. The fact that God sent His Son into the world is emphasized in many Scriptures (e.g., I John 4:9-10). That He came to save the world from the condemnation it deserved is also confirmed in many other Scriptures (e.g., John 5:24; Acts 17:31; Romans 8:1).
3:18 only begotten. See on John 1:18 for the vital importance of believing in the “only begotten” Son of God. His unique Sonship required the virgin birth, and was proved by the resurrection (Psalm 2:7; Acts 13:33; Romans 1:4; John 5:26).