The theme of the song is about a love so strong that nothing ... "no mountain high enough ... no valley low enough ... no river wide enough ... to keep me from you" ... can separate the love between them.
But this theme is not just a song for those of us who know the love of Jesus Christ ... this truth of a love so strong is one of God's gracious love for us, that He gave His only Son for our redemption (John 3:16) and the depth of His love.
35 Who shall separate us from the love of Christ? shall tribulation, or distress, or persecution, or famine, or nakedness, or peril, or sword?
38 For I am persuaded, that neither death, nor life, nor angels, nor principalities, nor powers, nor things present, nor things to come,
39 Nor height, nor depth, nor any other creature, shall be able to separate us from the love of God, which is in Christ Jesus our Lord