The heart of Reformed Theology or Calvinism is the cross of Jesus Christ … that all redemptive blessings flow from Him alone (Eph 1:3). That His Person and work is sufficient. That salvation is all of grace because it is all of Christ. It is a sign of a corrupted doctrine which teaches anything in addition to Christ …i.e. that man must (at least partly) either attain or maintain his own just standing before God. Now you either believe one or the other.. If you believe that Jesus is not sufficient to save you to the uttermost, then you embrace a theology to a greater or lesser degree like Roman Catholicism (Gal 3:3).
One of the main purposes of the five points of Calvinism is to demonstrate that the Scripture would always turn our trust entirely back to Christ: if you reject say, irresistible grace, then you then you reject or downplay the sufficiency of Christ, who provides EVERYTHING we need for salvation, including a new heart to believe (see Deut 29:4, 30:6; Ezek 36:26; John 6:63, 65, 37; 1 Pet 1:3). The same goes for preservation of the saints (which Jesus also teaches ex. John 6:37-40). If one believes they can lose salvation it is like saying that what Jesus did was not really sufficient … THAT WE must (at least partly) maintain our own just standing before God… So the five points (which are all grounded in Scripture) are simply a way to demonstrate that “salvation is of the Lord” … that it is “because of him you are in Christ Jesus, who became to us wisdom from God, righteousness and sanctification and redemption, so that, as it is written, “Let the one who boasts, boast in the Lord.” (1 Cor 1:30-31)