Ephesians 2:8 For by grace you have been saved through faith. And this [referring to salvation through faith] is not your own doing; it is the gift of God, 9 not a result of works (not founded upon anything in the believer himself), so that no one may boast. 10 For we are His workmanship, created in Christ Jesus for good works, which God prepared beforehand so that we would walk in them.
“And this is not your own doing.” The word “this” must have an antecedent, which would normally be the closest preceding noun. In this case, “this” would refer back to “faith.” Paul is not saying that grace is not our own doing. That would be redundant, because if it were our own doing, it would not be gracious at all. Rather, he says that faith is not our own doing. That does not mean that faith is not found in us; it is found in us. It does not mean that it is someone else’s faith by which we are justified. It is, properly speaking, our faith, for we are the ones who have it, the ones who are exercising it. But it is not our own doing, meaning that we are not the origin of it. It is not something that we have generated by our own power, nor does it originate in our flesh. R.C. Sproul Truths We Confess
Furthermore, if synergism is embraced, then there is the very real but subtle danger that men could boast that they made use of God's grace or had more wisdom than the man who rejected Christ. They could boast that they are different for, unlike others, they responded to Christ. The autonomous natural man would, then, ultimately determine His own salvation, not God. Since a work is a purpose achieved by physical or mental exertion. To believe is a difficult task; the Bible says it is foolishness to the unsaved and no one seeks God. Finally, the context of the verse (2:1-10) forbids the idea that man has any positive role in his own salvation. The verses include expressions like, "by grace you have been saved," "this not from yourselves," "it is the gift of God," "not by works," "so that no one can boast," "we are God's work," "created in Christ Jesus to do good works," "which God prepared in advance for us to do."
The above is not my words, but my sentiments expressed succinctly. I the my/simplest words: The gift of 'Faith' comes from God who is the cause. The source of 'faith' in not the individual. The effect coming from cause (God) is that Christians exercise/manifest the gift of 'faith' given them. In other words, regeneration (born again) logically precedes conversion (faith + repentance).