I have been discussing this issue with a Jehovah's Witness, this is my response to a post she made.
Allow me to give you my reasoning for believing in eternal security (once saved always saved).
First lets look in James 2. Beginning in 14 James writes, "What does it profit, my brethren, if someone says he has faith but does not have works? Can faith save him?" He then continues on to describe a scene of a naked and hungry brother or sister (in the faith), and a “believer” that says to them, "Depart in peace, be warmed and filled." However this “believer” does nothing for them, this is showing that the believer has “faith” but no works to back up the faith. “But do you not know, O foolish man, that faith without works is dead?” In verse 18 it is written, "Show me your faith without your works, and I will show you my faith by my works."
Thus works are the overflow of faith in the believers life, works alone cannot save one, but one can be justified by faith through works, "Do you see that faith was working together with his works, and by works faith was made perfect?"
Another passage that I stand firmly in is John 10:28-30, "And I gave them eternal life and they shall never perish: neither shall anyone snatch them out of My hand. My Father, who has given them to Me, is greater than all; and no is able to snatch them out of My Father's hand. I and My Father are One." “No one is able to snatch them out of My hand.” Meaning no one can take one’s salvation away from one, not even the one that has the salvation; because My Father who is greater than all holds them tightly in His grip.
Now I cannot espouse to think that I can overcome the creator of the Universe in saying, “I can pull myself out of His omnipotent grip.” That would be making God nothing more than a weak man, which is nothing short of blasphemy.
Yet another verse is that of Romans 8:30, “Moreover whom He predestined these He also called; whom He called, these He also justified; and whom He justified, these He also glorified (sanctified).” Not only does this verse present God as the initiator of faith, but also the finisher of the work of salvation. Revealing that there is a plan for those that are “called” from beginning to end. Not a plan that is contingent on the mere whim of mortal, broken, fallen man, but a plan that relies wholly on the Immortal, Whole, Creator of the Universe.
One final verse that I feel speaks completely for itself is John 6:39, “This is the will of the Father who sent Me, that of all He has given Me I should lose nothing, but should raise it up at the last day.”
To take this one step farther, this just makes sense. It is logical. That God would begin and finish the plan of salvation, instead of leaving it up to a fallen people of debase minds. We praise God as great, and worthy of praise, the Creator, the Savior by Whom grace came and saved, mercy was poured out, and salvation was made possible. But we would not dare to give God the glory that He is due for being able to hold us firmly in His grip! “He holds my life! And I praise Him for that, but He cannot hold my salvation!” That is blasphemy!