What do we mean by that?
J.I. Packer sums it up: “To Calvinism there is really only one point to be made in the field of soteriology: the point that God saves sinners.”
Packer is saying that salvation is entirely of the Lord, and sinners have nothing to do with their salvation. That God saves you out of his own good pleasure as an act of his delight. Sinners do not save themselves in any sense at all. Every step is an act of grace. Salvation is entirely an act of God.
What room does this gospel of grace leave you to boast? What room does it leave you for self-promotion? What need do you have to prove yourself to God and others? If what Paul writes is true, you have none.
If you really understand this gospel, this message that “God saves sinners,” and really understand Reformed soteriology, then you should be known for your humility, not your pride. You know that everything you have is a gift of grace.