Key Verses: John 3:16 For God so loved the world, that
he gave his only begotten Son, that whosoever believeth in him should not
perish, but have everlasting life.
17 For God sent not his Son into the world to condemn the world; but that the
world through him might be saved.
18 He that believeth on him is not condemned: but he that believeth not is
condemned already, because he hath not believed in the name of the only begotten
Son of God.
John 10:27 My sheep hear my voice, and I know them, and they follow
me:
28 And I give unto them eternal life; and they shall never perish, neither shall
any man pluck them out of my hand.
29 My Father, which gave them me, is greater than all; and no man is able to
pluck them out of my Father's hand.
1 John 5:10 He that believeth on the Son of God hath the witness in
himself: he that believeth not God hath made him a liar; because he believeth
not the record that God gave of his Son.
11 And this is the record, that God hath given to us eternal life, and this life
is in his Son.
12 He that hath the Son hath life; and he that hath not the Son of God hath not
life.
13 These things have I written unto you that believe on the name of the Son of
God; that ye may know that ye have eternal life, and that ye may believe on the
name of the Son of God.
One of the greatest areas of division between Baptists and other denominations, such as Methodists, is whether or not salvation is a permanent state. I grew up in the United Methodist Church, and was taught from childhood that salvation could be gained and lost repeatedly. The belief was so entrenched that it took me several years of being a Baptist to accept the truth that my relationship to God as his adopted son was a permanent one. Those who accept the notion that we can be saved, then not saved, then saved again, ultimately believe in salvation by works. Though they would protest that they do, in fact, believe in salvation by grace, that belief is belied by one that says we can lose that same salvation by the works we do. In one of my discussion classes in the Methodist Church, there was a prolonged discussion about whether or not a Christian who had sinned, then was killed before asking forgiveness, would still go to Heaven. The ultimate expression of this doctrine passes the ridiculous; salvation becomes so fluid that it can be lost and gained countless times, and the believer can never have assurance of salvation.
Arianism, the belief that we can approach God through human efforts, runs counter to the clear teaching of the Gospel that the Way to salvation lies only through Jesus Christ. Salvation is based on a relationship, our position as children, not on keeping the requirements of a contract. It is certainly true that God is not pleased when we disobey, and he will chasten us as his children, but just because we do something wrong doesn't mean he disavows us as his children. It would be equivalent to our renouncing our parental rights to our children every time they misbehave, then taking them back when they say they're sorry and ask for forgiveness.
Christ gives us the assurance that we have eternal life at the point we accept him as Savior, and follow him. Nowhere in Scripture is there any indication that eternal life, once it is given, can be taken back, or that someone can be saved more than once. The only sin which cannot ultimately be forgiven is final rejection of Christ, but once we belong to him, he gives us the assurance we cannot be "plucked out of his hand". Those apostates and reprobates who deny Christ and reject salvation with full knowledge of the Gospel were never saved to begin with; they are those "wolves in sheep's clothing" who pretend to be part of the body of Christ but are none of his.
Those who say we can't know if we'll enter Heaven until we get there also deny the truth of Scripture. The gift of eternal life is ours when we are saved; it is never something we "earn"; Christ has already paid the price for it. If we could get to Heaven by having our deeds weighed in some divine scale, Christ died in vain. If we can't gain salvation by anything we do, then how could we lose it by the things we do? Sin and disobedience have consequences for Christians, but losing the unearned gift of eternal life is not one of them.