My answer is unequivocally Yes! IF after claiming to "believe in Jesus Christ" he STILL maintained the DOCTRINE and views taught and held about Jesus Christ by Judaism, Hinduism or the Sikh's AFTER comparing the biblical doctrine of the person and work of Christ with that of Judaism, Hinduism or the Sikh's. A study of their doctrine clearly reveals that neither Judaism, Hinduism or the Sikh's believe any of the following BIBLICAL DOCTRINE regarding the Lord Jesus Christ: that Jesus Christ is not a created being but the creator of the universe and everything it contains, the Son of God, God incarnate, the Messiah of Israel and of the entire world, THE exclusive Savior of the world, the ONLY WAY to God the Father, that He was born of a virgin, that He died and paid the price for ALL of the sins of ALL of mankind for ALL of human history, that he bodily arose again from the dead and bodily ascended into heaven where He now sits at the right hand of God, until His enemies be made a footstool for His feet, that He is coming again, that He will destroy all of His enemies and reign upon the throne of David as the supreme ruler of the entire world for a thousand years and then, He will reign forever from the New Jerusalem.
Since Judaism, Hinduism, and the Sikh's do not believe ANY of the above BIBLICAL DOCTRINE, regarding the person and work of the Lord Jesus Christ. It stands to reason that if any Jew, Hindu or Sikh were to say that they "believe in Jesus", but they still stauncly adhere to what Judaism, Hinduism or the Sikh's believe and teach concerning Jesus Christ it is clear that they believe in another a different "jesus", they are NOT Christians and that they do NOT believe in the Lord Jesus Christ of the Bible.
Unfortunately, Jesus and the Apostles forewarned us that this would come and that there would be many false (other), Christs, and many false ("another") "Jesus'" along with many false Prophets, and many false teachers who would bring in "destructive heresies", denying the TRUE JESUS who paid for their sin upon Calvary:
"For false Christs and false prophets will arise and will show great signs and wonders, so as to mislead, if possible, even the elect." (Matt. 24:24; Mark 13:22)
"false Christs and false prophets will arise, and will show signs and wonders, IN ORDER TO LEAD ASTRAY, if possible, the elect." (Matt. 24:24; Mark 13:22)
"MANY false prophets will arise and will MISLEAD many." (Matt. 24:11)
"But false prophets also arose among the people, just as there will also be FALSE TEACHERS among you, who will secretly introduce destructive heresies, even denying the Master who bought them, bringing swift destruction upon themselves." (2 Peter 2:1)
What if he was Catholic?