Rom 12:1 I beseech you therefore, brethren, by the mercies of God, that ye present your bodies a living sacrifice, holy, acceptable unto God, which is your reasonable service.
Rom 12:2 And be not conformed to this world: but be ye transformed by the renewing of your mind, that ye may prove what is that good, and acceptable, and perfect, will of God.
This is speaking to the children of God for Paul called them brethren for when they are saved they are to grow in Christ, and to be like Christ representing goodness which is what this is speaking about.
Mat 24:50 The lord of that servant shall come in a day when he looketh not for him, and in an hour that he is not aware of,
Mat 24:51 And shall cut him asunder, and appoint him his portion with the hypocrites: there shall be weeping and gnashing of teeth.
Rom 11:20 Well; because of unbelief they were broken off, and thou standest by faith. Be not highminded, but fear:
Rom 11:21 For if God spared not the natural branches, take heed lest he also spare not thee.
Rom 11:22 Behold therefore the goodness and severity of God: on them which fell, severity; but toward thee, goodness, if thou continue in his goodness: otherwise thou also shalt be cut off.
But if they do not move forward and be Christlike they will not be brethren anymore for Jesus said the hypocrite will not be with Him.
2Ti 3:4 lovers of pleasures more than lovers of God;
2Ti 3:5 Having a form of godliness, but denying the power thereof: from such turn away.
2Ti 3:6 For of this sort are they which creep into houses, and lead captive silly women laden with sins, led away with divers lusts,
2Ti 3:7 Ever learning, and never able to come to the knowledge of the truth.
Some people did not repent correctly when they confessed Christ and never came to the truth.
2Pe 2:20 For if after they have escaped the pollutions of the world through the knowledge of the Lord and Saviour Jesus Christ, they are again entangled therein, and overcome, the latter end is worse with them than the beginning.
2Pe 2:21 For it had been better for them not to have known the way of righteousness, than, after they have known it, to turn from the holy commandment delivered unto them.
2Pe 2:22 But it is happened unto them according to the true proverb, The dog is turned to his own vomit again; and the sow that was washed to her wallowing in the mire.
Some people repented correctly but went back to sin and did not continue in the goodness of God.
Rom 12:1 I beseech you therefore, brethren, by the mercies of God, that ye present your bodies a living sacrifice, holy, acceptable unto God, which is your reasonable service.
Rom 12:2 And be not conformed to this world: but be ye transformed by the renewing of your mind, that ye may prove what is that good, and acceptable, and perfect, will of God.
So Paul is telling the ones that repented correctly to move forward acting like Christ and growing in Christ which is the perfect will of the Father.