See Rom 11:7-10.

This was Saint Paul explanation as to why and how salvation came to the Gentiles, as continued in v.11 “Hence I ask, did they stumble so as to fall? Of course not! But through their transgression salvation has come to the gentiles, so as to make them jealous.” Also Paul made referenced to the Old Testament, in which yes the Lord ]was selective, as only Jews could perceive the truth. But that was broken by the advent of Christ, who came to enlightened and save both Jews and Gentiles, that all who come may be saved (note the word all)
See Rom 5:20-6:2.

Paul explanation that the grace of God outmatches the productivity of sins, however it does not justify once saved always saved. As for chapter 6, Paul made an assertion that since Christians are born again through Christ, there is no point of sinning and return to the enslavement of transgression
That’s exactly what Jesus says (Mt 10:22), He also says that the Holy Spirit is given as a
guarantee of final salvation (Eph 1:13-14, cf 2 Cor 1:22), and He also says that He who causes men to born again,
He also protects those men by His power through faith for salvation (1 Pet 1:3-5).
Do you believe that?
I also believe man has a freewill to chooses to depart from God if he wills it. The Holy Spirit promises protection, but it does not mean the person cannot leave God by his own desire, we see this many time with folks who accept Christ and leave the faith