Jehovah's Witnesses
Jehovah's Witnesses certainly have lots of zeal, as I am hard-pressed to think of another denomination that does as much door to door witnessing as they do.However, as the Apostle Paul said that if what we believe is wrong (or IF our faith is misplaced), then we may very well miss the mark, and miss out on eternity, simply because we were trying to do it in our own strength, or because we thought that we could EARN God's Gift of eternal life, or because we thought that "doing" religious things equated with doing God's will for our lives, or with knowing the Lord in a personal way. See Matt. 7:21-23; Romans 10:1-4, 13; and Phil. 3:1-14. Note also that the "false circumcision" would be people who put their faith in the physical act of circumcision (or water baptism), but whose hearts are uncircumcised, and hard toward God, and the things of God. Gal. 2:1-21; 3:1-29; 5:1-6; Deut. 10:16
One of the main problems I have with Jehovah's Witnesses is that they don't believe that Jesus Christ was, (and is) God in the flesh: not God the Father, but God the Son, and that before He became a man, He was just like God the Father -- with the same powers, and made of the same essence, and who also existed from eternity past. See Phil. 2:5-11; Col. 1:15-19 (note that "first-born" is not the same thing as "first-created" -- and that the word in the Greek is "first-born": meaning the Chief, or Ruler, or one who will Inherit the Kingdom). See also Micah 5:2 and John 1:1-3 and note that Jesus was God, and was also with God in the beginning. Also note that His goings forth are from the day's of eternity. See also Hebrews 1:1-14 and note that Jesus is NOT an angel, but something more... and also that God the Father calls Him God: i.e. "Thy Throne O God, is forever and ever." vs. 8.
And that is what I think of Jehovah's Witnesses.