Sanctification can be a confusing subject. Nonetheless, it should not be overlooked or minimized. The danger is that the concepts of sanctification and Matthew 7:21 may easily be confused with one another if we are not careful. They are not interchangeable. They are not the same. But they do impinge upon one another.
Matthew 7:21 is by grace. It is a gift of God for all those who will believe and embrace it:
For by grace you have been saved through faith, and this is not your own doing; it is the gift of God — not the result of works, so that no one may boast. (Ephesians 2:8-9)
People who believe in that Jesus died for us to pay the penalty of our sins and who accept his gift are saved:
But what does it say?
“The word is near you,
on your lips and in your heart”
(that is, the word of faith that we proclaim); because if you confess with your lips that Jesus is Lord and believe in your heart that God raised him from the dead, you will be saved. For one believes with the heart and so is justified, and one confesses with the mouth and so is saved. (Romans 10:8-10)
Notice a key element here. We must accept that Jesus is our Lord and confess it. There is no such thing as cheap grace. We must participate in the gift of grace:
Not everyone who says to me, ‘Lord, Lord,’ will enter the kingdom of heaven, but only the one who does the will of my Father in heaven. (Matthew 7:21)
For just as the body without the spirit is dead, so faith without works is also dead. (James 2:26)
As Christian disciples we are required to grow in our faith. This is where the concept of sanctification comes in. God produces the growth, but we must cooperate with him. The Apostle Paul writes:
Therefore, my beloved, just as you have always obeyed me, not only in my presence, but much more now in my absence, work out your own salvation with fear and trembling; for it is God who is at work in you, enabling you both to will and to work for his good pleasure. (Philippians 2:12-13)
What is the purpose of sacntification? Holiness.
Therefore, prepare your minds for action, keep sober in spirit, fix your hope completely on the grace to be brought to you at the revelation of Jesus Christ. As obedient children, do not be conformed to the former lusts which were yours in your ignorance, but like the Holy One who called you, be holy yourselves also in all your behavior; because it is written, “YOU SHALL BE HOLY, FOR I AM HOLY.” (1 Peter 1:13-16)
We grow in holiness and righteousness. Do we dare take it lightly? Not if we want to be ready for the rapture of the bride of Christ.
Husbands, love your wives, just as Christ loved the church and gave himself up for her, in order to make her holy by cleansing her with the washing of water by the word, so as to present the church to himself in splendor, without a spot or wrinkle or anything of the kind—yes, so that she may be holy and without blemish. (Ephesians 5:25-27)
Since all these things are to be dissolved in this way, what sort of persons ought you to be in leading lives of holiness and godliness, 1waiting for and hastening the coming of the day of God, because of which the heavens will be set ablaze and dissolved, and the elements will melt with fire? 1But, in accordance with his promise, we wait for new heavens and a new earth, where righteousness is at home. Therefore, beloved, while you are waiting for these things, strive to be found by him at peace, without spot or blemish; (2 Peter 3:11-14)