The Bible is so clear on whether or not you can lose eternal life it's amazing to me that OSAS even exists in the church:
The teaching exists because it's found in the Bible, and quite clearly.
First, Paul described both justification (Rom 3:24, 5:15,16,17) and eternal life (Rom 6:23) as gifts of God. Then he wrote Rom 11:29 - the gifts and calling of God are irrevocable. Since he had already described what he meant by "gifts of God", there was no reason for him to specifically list what he meant by "gifts of God".
3:24 - being justified as a gift by His grace through the redemption which is in Christ Jesus
6:23 - For the wages of sin is death, but the free gift of God is eternal life in Christ Jesus our Lord.
11:29 - for the gifts and the calling of God are irrevocable.
Second, every believer is sealed with the Holy Spirit when they believe (Eph 1:13). This sealing is a pledge with a view to the redemption of God's own possession (believers - Eph 1:14).
And, this sealing is for the day of redemption (Eph 4:30).
1:13 - In Him, you also, after listening to the message of truth, the gospel of your salvation—having also believed, you were sealed in Him with the Holy Spirit of promise
1:14 - who is given as a pledge of our inheritance, with a view to the redemption of God’s own possession, to the praise of His glory.
4:30 - Do not grieve the Holy Spirit of God, by whom you were sealed for the day of redemption.
Third, Jesus tells us WHEN one HAS eternal life; when they believe (Jn 5:24). Then, He tells us that those to whom He gives eternal life WILL NEVER PERISH (Jn 10:28).
5:24 - “Truly, truly, I say to you, he who hears My word, and believes Him who sent Me, has eternal life, and does not come into judgment, but has passed out of death into life.
10:28 - and I give eternal life to them, and they will never perish; and no one will snatch them out of My hand.
Fourth, Paul stated that regardless of the believer's lifestyle, or "whether we are asleep or awake, we will be together with Him" in 1 Thess 5:10. The context begins in v.4 and contrasts believers with unbelievers, or day with night, or being alert with being asleep or sober with drunkeness.
4 But you, brethren, are not in darkness, that the day would overtake you like a thief;
5 for you are all sons of light and sons of day. We are not of night nor of darkness;
6 so then let us not sleep as others do, but let us be alert and sober.
7 For those who sleep do their sleeping at night, and those who get drunk get drunk at night.
8 But since we are of the day, let us be sober, having put on the breastplate of faith and love, and as a helmet, the hope of salvation.
9 For God has not destined us for wrath, but for obtaining salvation through our Lord Jesus Christ,
10 who died for us, so that whether we are awake or asleep, we will live together with Him.
Analysis of this passage:
v.4 tells us that believers are "not in darkness"
v.5 differentiates believers (sons of light and day) with unbelievers (not of night or darkness).
v.6 encourages believers to not live like unbelievers (not sleep as others do, but be alert and sober).
v.7 describes unbelievers and what they do.
v.8 explains that "since we are of the day" (believers), we need to be sober.
v.9 explains the destiny of the believer - not destined for wrath but for salvation
v.10 says that regardless of the believer's lifestyle, we will live together with Him.
Fifth, Jesus noted how people are saved in John 10:9 - “I am the door; if anyone enters through Me, he will be saved, and will go in and out and find pasture.
The Greek word for “enters” is in the aorist tense, meaning “in a point in time”, as opposed to the present tense, which those who believe in loss of salvation only emphasize. Iow, one must continue to believe in order to continue to have eternal life. Further, Paul used the aorist tense in his answer to the jailer in Acts 16:31, and Jesus used the aorist tense in Luke 8:12 “believed and be saved”.
Sixth, there are absolutely zero verses that warn us plainly that one can lose their salvation.
The first 5 points teach eternal security. And do so clearly. The 6th point is just icing on the cake.
However, if the passages in the first 5 points teach something else, I'd be interested to know what they do teach.
One thing I know; they teach something. I believe they are very clear about eternal security and cannot imagine what else they could be teaching.