John 5:24
Verily, verily, I say unto you, He that heareth my word, and believeth him that sent me, hath eternal life, and cometh not into judgment, but hath passed out of death into life.
ASV

These are the words of Jesus as He spoke to several Jews who were seeking to kill Him.  They were incensed because Jesus made the claim that God was His Father as quoted in verse 17.  What a slap in the face this was to the Jews when Jesus told them they had to believe in God who had sent Him to proclaim these things. 

This verse of scripture is a sugar stick for those who advocate salvation by faith alone.  They point to the word "believe" and draw the conclusion that this means that a mental assent of the Godhead is all that is required for one to be saved.  If this were true, one would therefore not have to repent of their sins or confess Jesus Christ as the Son of God before men.  Jesus Himself so stating in Luke 13:3-5, Matthew 10:32-33 and Luke 12:8-9.  Faith plus anything cannot be faith alone and those who would defend this false doctrine bear the burden of demonstrating from scripture how one can be saved without repentance. confession and baptism when the scriptures explicitly state that all three of these things are required, (Luke 13:3, Romans 10:10, John 3:5). 

Those who hold to this false doctrine do not have a correct understanding of how the word for "believe" is used here and in other places.  When the stated need to believe is used, the other things commanded by Christ are implied.  One cannot claim to believe in Christ or God, but reject their teachings or their commandments.  For instance, Jesus said in Luke 13:3 that all those who do not repent will perish.  If we believe in Christ, we must also believe what He taught about repentance.  To reject His teaching on repentance is to disbelieve Him.  The same thing applies for confessing Him as the Son of God, baptism, worship in spirit and in truth, evangelizing and works of benevolence. 

To illustrate, say a boy finds a heavy safe that could be full of treasure but he is not strong enough to bring it home by himself.  He excitedly goes and tells his father about it and pleads with him to come help bring the heavy strongbox home.  The father, being skeptical is hesitant to commit to it, thinking his son is out of his mind.  His son, pleading with his father makes this statement, "Dad, if you will just believe me, we can be rich".  When the young man made that statement to his father, it was understood by both of them that a call for action was implied in the plea for belief. 

The biggest hurdle the early Christians had in bringing people to Christ was to get them to believe that He was who He said He was.  In the stated need to believe, it was understood by all that a call to action was implied.  Failure to act was failure to believe.  

Those today who advocate salvation by faith alone are not realizing that salvation is either by faith alone, or it isn't.  If just one thing more than a mental assent is necessary, then salvation is not by faith alone.  And when one comes to the realization that salvation is not by faith alone, then all the other things Jesus commanded come into play as well.  And well it should because in this very context, during this very teaching of Christ as recorded in verses 28-29, we read these words: "Marvel not at this: for the hour cometh, in which all that are in the tombs shall hear his voice, and shall come forth; they that have done good, unto the resurrection of life; and they that have done evil, unto the resurrection of judgment." (John 5:28-29).  If belief alone were what Jesus was teaching in this context, then why oh why would He have declared that only those who do good will be resurrected to life?  We must always remember to allow the overall context to help define the words of scripture.  And from there, we must allow the overall teachings of the new covenant to define the context.  Since all of God's word is truth, we must never understand a verse in such a way that the rest of scripture can not agree with. These words of our savior should be enough to forever quell the voices of those who would try and say that faith alone is all that is required. 

The second chapter of James directly addresses the fallacy of this belief.  Evidently there were those in the first century who also mistakenly concluded that faith alone was the ticket to heaven.  James flatly condemns this in words which are impossible to deny: James 2:20-24, "But wilt thou know, O vain man, that faith apart from works is barren? Was not Abraham our father justified by works, in that he offered up Isaac his son upon the altar? Thou seest that faith wrought with his works, and by works was faith made perfect; and the scripture was fulfilled which saith, And Abraham believed God, and it was reckoned unto him for righteousness; and he was called the friend of God. Ye see that by works a man is justified, and not only by faith." (ASV)