God grants initial salvation by faith apart from works (Ephesians 2:8-9), just as infants are born apart from their works. But just as an infant after it is born needs to begin to breath, and then to continue to breathe, if it is to remain alive, so a new believer after he is born again needs to begin to perform works of faith (1 Thessalonians 1:3, Galatians 5:6b) (not works of the letter of the Old Covenant Mosaic law), and to continue to perform them (Titus 3:8), if his faith is to remain alive (James 2:26).
For faith is like a body, and works of faith are like the breathing (spirit) of that body (James 2:26). Faith without works of faith will die, just as a body without breathing will die (James 2:26). That is why our ultimate salvation will depend on both our faith and our continued works of faith (Romans 2:6-8, James 2:24, Matthew 7:21). If a believer refuses to continue to perform works of faith, without repentance, he will ultimately lose his salvation (Matthew 25:26,30, John 15:2a), just as if someone stops himself from breathing by hanging himself, he will die.
The breathing analogy (James 2:26) does not include the automatic aspect of breathing. For believers must be careful to maintain good works (Titus 3:8). The analogies in the Bible do not include every aspect of the analogous thing. For example, believers, born-again people, being like newborn babies (1 Peter 2:2) does not mean that believers have no ability to talk, walk, or control their bowels.
That brings to mind Calvinism's mistaken doctrine of once-saved-always-saved through assured perseverance. This doctrine unwittingly ends up logically requiring that saved people are robots. For if saved people cannot choose to do evil to the point where they can ultimately lose their salvation, then they no longer have free will. Also, the mistaken doctrine of assured perseverance unwittingly ends up logically requiring that a Christian can have no present assurance that he is truly saved. For if a Christian who does not persevere to the end was never truly saved, then no Christian can presently have the assurance that he is truly saved. For no Christian can know if he will persevere to the end. Down the road, he could fail to persevere, and so end up showing that he was all along only a fake Christian, a self-deceived hypocrite.
But under true, Biblical doctrine, every believer in the gospel (of 1 Corinthians 15:1-4, John 20:31) can know that he is presently saved (1 John 5:13; 1 Corinthians 15:1-4), if after he became a believer in the gospel he repented from his sins (1 John 3:6) and confessed them to God (1 John 1:9). And he can be sure that as a saved person, he can never be separated from the love of God, so long as he loves God (Romans 8:28-39), which means to obey him (1 John 5:3, John 14:21-24). And no matter how many tests a saved person fails during his lifetime, sometime subsequent to his initial repentance, even if he fails and commits sin seventy times seven times in a single day (Matthew 18:21-22, Luke 17:4), he can be sure that so long as he sincerely repents from every act of sin and confesses it to God, he will be completely forgiven (1 John 1:9). He will lose his salvation ultimately only if he wrongly employs his free will to do something like committing a sin without repentance (Hebrews 10:26-29; 1 Corinthians 9:27, Luke 12:45-46), or becoming utterly lazy without repentance (Matthew 25:26,30, John 15:2a, Romans 2:6-8), or committing apostasy (Hebrews 6:4-8, John 15:6; 2 Timothy 2:12).
We are initially saved now, but not yet ultimately saved.
That is, the difference between initial salvation and ultimate salvation is that initial salvation is the salvation which Christians have now (Ephesians 2:5) in their mortal bodies, while ultimate salvation is that salvation which is ready to be revealed in the last time (1 Peter 1:5), and is always drawing nearer (Romans 13:11), that salvation which Christians are still hoping for (1 Thessalonians 5:8, Romans 8:23-25, Mark 10:30), and which Jesus will bring to obedient Christians at his second coming (Hebrews 9:28, Hebrews 5:9), when he will resurrect (if dead) or change (if alive) their mortal physical bodies into immortal physical bodies just like the immortal physical body which Jesus obtained at his resurrection on the third day after his death (Luke 24:39,46; 1 Corinthians 15:3-4,21-23,51-53, Philippians 3:21, Romans 8:23-25, Philippians 3:11-14).