Notice how John 3:16 is rendered in the New American Standard Bible:
"For God so loved the world, that He gave His only begotten Son, that whoever believes in Him shall not perish, but have eternal life."
Most Bibles use the word "believe" here. Now notice John 3:16 in the New World Translation Bible:
"For God loved the world so much that he gave his only-begotten Son, in order that everyone exercising faith in him might not be destroyed but have everlasting life."
Some have condemned the New World Translation for rendering `exercise faith' here instead of `believe.'" Yet the word usually translated `believe' in many Bibles (pisteuo) may honestly be rendered `exercise faith', which implies certain action on the part of the believer. For example, the highly reputed (and highly "orthodox") Kittel's Theological Dictionary of the New Testament, (Abridged in One Volume) tells us:
I. The OT Legacy. OT faith corresponds to Gk. pisteuein inasmuch as both involve trust in persons and belief in words (including God and his word). The OT term, however, carries a stronger element of acknowledgment and obedience. - p. 852.
II. General Christian Usage.
1. Continuation of the OT and Jewish Tradition.
 b. pisteuo as "to obey." Heb. 11 stresses that to believe is to obey, as in the OT. Paul in Rom. 1:18; 1 Th. 1:8 (cf. Rom. 15:18; 16:19) shows, too, that believing means obeying. He speaks about the obedience of faith in Rom. 1:5, and cf. 10:3; 2 Cor. 9:13. - pp. 853, 854, Eerdmans Publ., 1985.
And Thayer's Greek-English Lexicon of the New Testament, p. 511, also tells us:
[pisteuo] .... g. used especially of the faith by which a man embraces Jesus, i.e. a conviction, full of joyful trust, that Jesus is the Messiah .... conjoined with obedience to Christ - Baker Book House, 1977.
So, even some of the most respected New Testament scholars admit that pisteuo includes the idea of obedience with belief! This requires belief in and acceptance of Christ's sacrifice on our behalf and action on the part of the believer. There are many things true believers must do and must not do. Doing (or not doing the forbidden things) certainly does not earn salvation. But not acting in obedience to the word of God may well prevent you from receiving the free gift of salvation.
There is clearly and obviously an inseparable union between true faith and actions on the part of the true Christian. As James 2:14-26 tells us:
What use is it, my brethren, if a man says he has faith, but he has no works? Can that faith save him? .... Even so, faith, if it has no works, is dead, being by itself. .... You believe that God is one. You do well; the demons also believe and shudder. But are you willing to recognize, you foolish fellow, that faith without works is useless? .... For just as the body without the spirit is dead, so also faith without works is dead. - NASB.
Notice, "just as the body without spirit is dead, so also faith without works is dead"! Faith and works (or active obedience to the word of God) are just as inseparable as the body and spirit are inseparable in a living man! When this faith is spoken of, then, it certainly should not be considered dishonest to translate it with that understanding: "exercise faith." (See the FAITH study paper; Search For Bible Truths)
The following is from the 12/1/90 Watchtower, Questions From Readers:
Why does the New World Translation at times render the Greek word pi·steu´o as “believe” (like most translations) and at other times as “exercise [or put] faith in”?
This is done to reflect different shades of meaning that are expressed by the Greek word pi·steu´o.
For example, A Grammar of New Testament Greek, by James Moulton, notes that early Christians clearly recognized “the importance of the difference between mere belief . . . and personal trust.” Both these thoughts can be expressed using the Greek word pi·steu´o.
Often, the different shades of meaning of pi·steu´o must be discerned from the context. At times, though, different grammatical constructions help us to see what the writer had in mind. For example, if pi·steu´o is followed merely by a noun in the dative case, the New World Translation usually renders it simply as “believe”—unless the context indicates something different. (Matthew 21:25, 32; but see Romans 4:3.) If pi·steu´o is followed by the word e·pi´, “on,” it is generally rendered “believe on.” (Matthew 27:42; Acts 16:31) If it is followed by eis, “to,” it is usually translated “exercise faith in.”—John 12:36; 14:1.
This latter rendering (which reminds us that pi·steu´o is related to the Greek word pi´stis, “faith”) is in harmony with a comment in An Introductory Grammar of New Testament Greek, by Paul Kaufman. This work says: “Another construction which is common in the New Testament (especially in John’s Gospel) is ??????? [pi·steu´o] with ??? [eis] and the accusative case . . . The whole construction of ??? plus the accusative must be translated rather than attempting to translate the preposition ??? as an isolated word. Faith is thought of as an activity, as something men do, i.e. putting faith into someone.” - 12/1/90 Watchtower; Questions From Readers