Justification.... Sanctification....
There are TWO DIFFERENT issue here: Justification and Sanctification (narrow sense, both).
Let me use this analogy:
1. FIRST: On a given date, I was born. I was GIVEN life as a free gift, unearned, undeserved, unmertited, not even requested or willed - the miracle, the wonderful, mysterious GIFT of life (we might agree that actually happened about 9 months earlier, but let's proceed). At that point, I became alive because I was GIVEN life. I became a human being - with all that means, biologically and spiritually, all that means in terms of God and me. GIFT. G.I.F.T. This purely, solely, only, exclusively by mercy since prior to that, I did NOTHING. I thought nothing. I willed nothing. I sought nothing. I desired nothing. NO good works. GIFT. G.I.F.T. Mercy. M.E.R.C.Y. On a specific date - I was removed from my mother (C-Section) - unbreathing, unconscience - I had NOTHING to do with it. NOTHING. N.O.T.H.I.N.G. Gift. Mercy. No merits. No works. No will. Nothing in or from me. GIFT. MERCY. Someone ELSE is to be credited. Entirely. Wholly. Completely. MONERGISTIC. Life is mine - by grace, by mercy, from God, as a GIFT. I am a human being, with all that means - by grace, by mercy, from God, as a GIFT.
In the same way, God saved me (what Protestants mean here is justification - narrow sense). God GAVE me spiritual life, God caused me to be born AGAIN, now not only with physical life but with spiritual life, now I am not only the child of my parents but a child of God. This CHANGES my relationship to God, as a result solely, only, exclusively because of God's mercy, grace, favor; solely, only, exclusively because of what CHRIST has done as THE Savior; solely, only, exclusively because God GAVE me the GIFT of faith in Christ as my Savior: Sola Gratia - Solus Christus - Sola Fide. You know, the worst heresy ever, what Luther was excommunicated for, the view the RCC decided to split Christianity over. We believe this is MONERGISTIC, because CHRIST is the Savior - not me, not you, not the Pope, not Mary, not the RC Denomination. I'm NOT the Savior - in whole or in part - because the job is taken and He didn't blow it. What makes me a CHRISTIAN is that God's merciful, gracious GIFT of faith means I'm looking to CHRIST as THE Savior, not in the mirror as all my Catholic teachers taught me to do, many Catholic sermons taught me to do, as Catholics keep telling us we must do.
2. SECOND: Almost immediately after being born (well, maybe some months later, lol), my parents, my society and yes God called me to GROW. To mature. To become more loving, more caring, more righteous, more ethical, more serving. THIS is a process (unlike my conception). THIS is synergistic (unlike conception). GROWING to be more God like. GROWING in the directions that my parents, my society, my God call me: "Thou shalt be HOLY just as the Lord God is holy." "Thou shalt be PERFECT just as your Father in Heaven is perfect." "LOVE in exactly the same way as Christ loved us on the Cross." High callings! I'm not "there" yet. I'm still GROWING (well, I'd LIKE to say always growing..... sometimes I'm not, sometimes I even retreat). And I do so in large part because of God's EMPOWERING, not due to some innate homo sapiens ability.
In the same way, when God saves us (regenerates, justifies, gives life) there immediately follows the call to grow, mature, to become Christ-like, to love exactly as and to the same extent that God loves, to be holy/sinless/perfect just as and to the same extent that God is. And we are empowered to do so, not just called to do so. No one has arrived there yet but hopefully we're growing up, growing in that direction.
The first is justification (narrow) - regeneration, born again, what Protestants mean by "salvation."
The second is sanctification (narrow) - discipleship, Christian living, maturing, acting out our faith, becoming Christ-like.
A lot of confusion is eliminated when we realize these are two different things, not to be confused.
Yes..... in a few cases, the Bible also calls this "grace" but the CONTEXT tells us this is different, here it means "strength" or "empowering". It is still ours by mercy (we don't DESIRE anything from Him), but here it means strength. This growing up, this discipleship, this CHRISTIAN-walk is something a CHRISTIAN does, not something that makes one a Christian; it is the RESULT of justification not the cause. My being nice to my neighbor is not what causes me to have physical life, having physical life enables me to be nice to my neighbor. What I do as a growing, maturing, developing man is not what makes me a homo sapiens nor worthy of being given life.