SOME REASONS WHY PEOPLE BELIEVE
THEY CAN LOSE THEIR SALVATION
Their church teaches it (traditions of men). We are to go by the Word of God as our final authority and NOT by man's opinions. Verses taken OUT OF CONTEXT and misapplied. Vague or unclear passages are used. Using verses that belong and deal with a different time period than for the church age. (Example: Quoting Matt. 24:13: which deals with the tribulation period).
Getting salvation and service mixed up for the Christian. Salvation means Grace as a FREE GIFT WITHOUT WORKS (Eph. 2:8, 9). Service deals with rewards, i.e. works; (I Cor. 3:11-15).
Following are seven reasons from the Bible why a born-again Christian cannot lose his salvation. Over 200 Scriptures are referenced, which are given at the end for ease of readability. Anyone reading this who doubts the truth of this doctrine, sometimes called "once saved, always saved", is encouraged to look up and read the Scriptures for himself.
1. Eternal Life - The Bible says that ETERNAL life is a present possession . If we could lose that life from God  then we never had ETERNAL life. Our life is Christ's life when we are saved  (see #4) and Christ cannot die again . I John 5:13 says we can KNOW that we have ETERNAL life. God promised eternal life and God cannot lie .
2. Born Again as Sons of God - When we accept Christ we are born again of God , are passed from death unto life , and become a new creature , or creation , with all things new. We would have to become unborn to lose our salvation and born a third time to get saved again, which is not mentioned in the Bible. We also become a son of God  with a new nature, Christ's nature , and Christ cannot deny himself . We get an incorruptible nature  and our will is changed . John 10:27 says the believer WILL follow Christ.
After we are saved, God no longer deals with us as sinners but as sons . He chastises but his chastisement is not payment for sin, it is child-training. God may chastise but the payment for ALL sins was made 2000 years ago on Calvary (see #3). Sometimes God may even chastise with death . The fellowship with God may change but not the Father-son relationship. For example, a father tells his boy not to play with the father's delicate tools or they will break. The boy disobeys and breaks the tools. As chastisement the father smacks the boy but the smack does not pay for the tools; the father still must pay for the new tools and the boy is still his father's son. All of our sins were paid for on Calvary and ALL of them are forgiven when we are saved (see #3).
3. What Happens to a Believer's Sins - In God's eyes when we get saved we are perfect forever , justified (made as if we had never sinned ), and sanctified , as far as our salvation is concerned. We still live in a sinful body and still sin but these only affect our fellowship with the Father once we are saved (see #2). The Bible says that God WILL NOT impute (charge up) sin to a believer  and that includes ANY sin. If a justified person could go to hell he was never truly justified. If after being saved a sin or sins could cause one to lose his salvation then all of his sins were not paid for at Calvary and forgiven (washed away) when he trusted Christ and his blood , as the Bible says . God forgives and forgets  every sin, including future sins (all our sins were future when Christ died for them), when we get saved.
4. What We Have "In" Jesus Christ - When we get saved we are placed into the body of Christ . We become "flesh of his flesh and bone of his bone" . Our soul is "circumcised" from our flesh with a circumcision "made without hands" . The Bible says that the body (church) will be presented without spot or blemish . It cannot be without blemish if pieces have to be removed. Our position is settled: we are mystically seated in heaven already , a joint-heir with Christ (a joint heir cannot be disinherited ), and safe in Jesus' and God's hand  where we cannot be plucked out. The Holy Spirit places us IN Christ  just as we were IN Adam  before we were saved. IN Christ we are safe from condemnation , accepted , preserved forever , sanctified , alive forever , a new creature , perfect , righteous ; and we have liberty , all spiritual blessings , and an inheritance . Our inheritance is reserved for us in heaven  (see #6), will not fade away , is incorruptible , and cannot be defiled .
Colossians 3:3 says our life is HID with Christ IN God; Satan has to get through God and then Christ to get to us, to cause us to be lost again. If Satan can overcome God and Christ to get one Christian, then he can get all of us because we are all sinners alike . If Satan knew there was a way to make a Christian lose his salvation he is subtle enough that he could cause every Christian to lose his salvation [43.1]. For a believer to go to Hell, Christ and God  would have to go to Hell, because we have everything in standing and position that the Lord Jesus Christ has and we are "in" Him . Our security is in a perfect person: Jesus Christ [45.1].