John 10:28 says:
"And I give unto them eternal life; and they shall never perish, neither shall any man pluck them out of my hand.", but its precedent verse says:
"My sheep hear my voice, and I know them, and they follow me:" So, only if we hear His voice, if we follow Him, no man will pluck us out of His hand. How do Christians follow Him? Here is how:
"If ye keep my commandments, ye shall abide in my love; even as I have kept my Father's commandments, and abide in his love."(John 15:10)
"This is my commandment, That ye love one another, as I have loved you." (John 15:12)

Here are some other verses which speak about the same matter.

Matthew 7

18 A good tree cannot bring forth evil fruit, neither can a corrupt tree bring forth good fruit.

19 Every tree that bringeth not forth good fruit is hewn down, and cast into the fire.
20 Wherefore by their fruits ye shall know them.

21 Not every one that saith unto me, Lord, Lord, shall enter into the kingdom of heaven; but he that doeth the will of my Father which is in heaven.

Hebrews 3

12 Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God.

13 But exhort one another daily, while it is called To day; lest any of you be hardened through the deceitfulness of sin.
14 For we are made partakers of Christ, if we hold the beginning of our confidence stedfast unto the end;

And many other verses (read the Bible...).
Let God be your guide!