John 5:24, Verily, verily, I say unto you, He that heareth my word, and believeth on him that sent me, hath everlasting life, and shall not come into condemnation; but is passed from death unto life.
John 6:47, Verily, verily, I say unto you, He that believeth on me hath everlasting life.
John 10:27-30, My sheep hear my voice, and I know them, and they follow me: And I give unto them eternal life; and they shall never perish, neither shall any man pluck them out of my hand. My Father, which gave them me, is greater than all; and no man is able to pluck them out of my Father's hand. I and my Father are one.
Hebrews 13:5-6, Let your conversation be without covetousness; and be content with such things as ye have: for he hath said, I will never leave thee nor forsake thee. So that we may boldly say, The Lord is my helper, and I will not fear what man shall do unto me.
In the first scripture, the life that is given is everlasting, and that means it can never have an end. They shall not come into condemnation, that is absolute terminology, as opposed to the should not perish of John 3:16 (kjv). Also they cross over from death into everlasting life the moment they believe; they do not have to wait for death in order to have everlasting life. Their last breath on earth is their first breath in heaven.
In the second verse, everlasting life is given to those who believe on Jesus Christ. They have it in the present tense.
In the third verse, They are given eternal life, they shall never perish, very absolute terminology; and no one can pluck them out of Christ's hand. If that weren't enough, the Father's hand is even more secure because He is Omnipotent, Omniscient, and Omnipresent. So there is strong security in His hand. And to be in Jesus' hand is to be in the Father's hand; because they are one and the same Person.
Finally, in the fourth verse, Jesus promises you and me that He will never leave us nor forsake us. That word never is also a pretty strong word and is absolute terminology.
Now in case anyone sees the loophole in the third verse (John 10:27-30) and wants to say that a person can walk away from God and thus leave God's hand; first of all I would ask, why do you want to contend for this idea? Do you want people who are truly born again to turn away from God and so you want to believe that it is possible?
But really, Jeremiah 32:38-40 deals with this loophole quite adequately impaho; although some try to contend that it is a promise given only to Israel. I would contend that this promise to Israel reveals the Lord's nature and character towards all of His people and therefore would apply to all of His people. Also, in context (v.38) the scripture is identified as a promise not to Israel only, but to God's people; which includes believing Gentiles. Not to mention the fact that believing Gentiles are graffed into that olive tree which represents Israel and is identified as Israel in the holy scriptures.