"Lose" is not entirely accurate. "Fail" will be more accurate.
The same argument could be made that if Grace is always irresistible then how can it be resisted? Yet, we know the Scriptures.
Heb 12:15 Looking diligently lest any man fail of the grace of God
It is my observation that most Calvinist will not engage unless you accept their terminology.
If we fail, then God failed. Jude 24-25. We are not the author of our own salvation. We did not begin a good work within ourselves. The Bible is clear that it is Jesus/God who began a good work in us, and it clearly states that He will be
faithful to complete it. So, if we fail, then God is unfaithful. Do you truly believe God is unfaithful? Irresistible grace means that those God has chosen for salvation
will be saved, and will not fail to be saved.
Hebrews 12:12-17 "12 Therefore lift your drooping hands and strengthen your weak knees, 13 and make straight paths for your feet, so that what is lame may not be put out of joint but rather be healed. 14 Strive for peace with everyone, and for the holiness without which no one will see the Lord. 15 See to it that no one fails to obtain the grace of God; that no “root of bitterness” springs up and causes trouble, and by it many become defiled; 16 that no one is sexually immoral or unholy like Esau, who sold his birthright for a single meal. 17 For you know that afterward, when he desired to inherit the blessing, he was rejected, for he found no chance to repent, though he sought it with tears."
So...what is the therefore there for? Don't remove a verse from context. This section is dealing with the discipline of God.