We're saved by Grace through Faith - but that faith obviously has to be continuous
Πίστευσον ἐπὶ τὸν κύριον Ἰησοῦν καὶ σωθήσῃ Act 16:31.
The commandment Believe! is aorist, not present tense there. Aorist is the tense used when one wishes to express action at a point in time, a single action. The promise is Believe on the Lord Jesus and you shall be saved. The following future result is salvation.
Something done over a period of time (not necessarily continuous for years) merits the present tense. And believing is something done not just at a moment, but over time. Thus John 3:16 implies that if now in the present time a man is believing, He receives eternal life, not temporary life with a shot at eternal life.
It has nothing to do with multi-point Calvinism, neither does mentioning the name "Calvin" prove anything.
And since salvation is instant, it does not require constant believing to receive salvation, though once a man does trust the Lord, there is no indication that He ever stops that basic trust for salvation, though he may fail to trust in some concrete situation.
Instant Salvation in Past Tense
“Your faith has saved you; go in peace.” (Luke 7:50)
th Zacchaeus.
Jesus said to him, “Today salvation has come to this house, because this man, too, is a son of Abraham.” (Luke 19:9)
Evidence of Zacc's instant salvation is found in his instant statement:
"But Zacchaeus stood up and said to the Lord, “Look, Lord! Here and now I give half of my possessions to the poor, and if I have cheated anybody out of anything, I will pay back four times the amount.” (Luke 19:8)
Cornelius was instantly saved when he trusted the Lord Jesus, and no water baptism was needed nor joining any organization. (see Acts 10:43-48). Compare this to what Peter said in Acts 15:11: We believe it is through the grace of our Lord Jesus that we are saved, just as they are. (Acts 15:11)
Consider Eph 2:5 [God] made us alive with Christ even when we were dead in transgressions—it is by grace you have been saved.
Is there any hint of an extended process between dead & alive? Salvation is a change from death to life, with no intermediate zombie state. 2 Cor 5: Old things are passed away, behold all has become new. It is all or nothing here.
In addressing Tim on common salvation, prophet Paul remarks to Tim how the Christian's salvation is past. There is no future to transpire before Paul and Tim should call themselves saved.
[the Lord God] who has saved us and called us to a holy life—not because of anything we have done but because of his own purpose and grace. This grace was given us in Christ Jesus before the beginning of time, - 2 Tim 1:9
Again, prophet Paul wrote in Titus the truth that we presently regard ourselves as saved in the past.
he [God] saved us, not because of righteous things we had done, but because of his mercy. He saved us through the washing of rebirth and renewal by the Holy Spirit, (Titus 3:5)
The future continued salvation state after belief is guaranteed by the past justification by the blood of the Lord Jesus.
Since we have now been justified by his blood, how much more shall we be saved from God’s wrath through him! (Rom 5:9) Now how is anyone going to be half-way justified? You either are or you are not. The transition must be instante.
Instant Salvation Is Realized in Present Time
John 5:24
Truly, truly, I say to you, whoever hears my word and believes [present tense] him who sent me has eternal life. He does not come into judgment, but has passed from death to life.
He who has the Son has life; he who does not have the Son of God does not have life. I write these things to you who believe in the name of the Son of God so that you may know that you have eternal life. (1 John 5:12,13)
There is a clear dichotomy: You either have or have not the Son of God with eternal life. The change must be instantaneous. Eternal life is a present possession for the believer, not just a future reward.
Salvation is clearly presented as a present reality:
Men are divided clearly into two groups, believers and non-believers, the saved & the perishing.
he who believes has everlasting life. (John 6:47)
For the message of the cross is foolishness to those who are perishing, but to us who are being saved it is the power of God. (1 Cor 1:18)
For we are to God the aroma of Christ among those who are being saved and those who are perishing. (2 Cor 2:15)
It is clear the whosoever believes in Him has everlasting life and does not come into condemnation. This salvation must be instantaneous in that it is a new birth, a recreation and only depends upon trusting the SAvior (not the chance-giver). One either is a saint or an ain't. The moment a non-believer becomes a believer, he must be saved, for scripture guarantees salvation to the believer. Believe on the Lord Jesus Christ, and you shall be saved. It is clear that the Philippian jailer was at once saved, for he believed.