Paul was writing to a pre-New Covenant body of believers. He was exhorting them to stay with their faith in Messiah Jesus (in lieu of the impending persecutions to the early church that were looming on the horizon for them), NOT to substitute good works for Jesus' atonement on the Cross.
You choose: Jesus' atonement, or your righteous acts, which are like filthy rags (Isaiah 64:6).
"For it is by grace you have been saved, through faith—and this not from yourselves, it is the gift of God— not by works, so that no one can boast."- St. Paul's Letter to the Ephesians, 2:8-9
Blessings!
James 2:14-18: What doth it profit, my brethren, though a man say he hath faith, and have not works? can faith save him?
15 If a brother or sister be naked, and destitute of daily food,
16 And one of you say unto them, Depart in peace, be ye warmed and filled; notwithstanding ye give them not those things which are needful to the body; what doth it profit?
17 Even so faith, if it hath not works, is dead, being alone.
18 Yea, a man may say, Thou hast faith, and I have works: shew me thy faith without thy works, and I will shew thee my faith by my works.
Hebrews 10:26 For if we sin wilfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins
Romans 8:12-13 Therefore, brethren, we are debtors, not to the flesh, to live after the flesh.
13 For if ye live after the flesh, ye shall die: but if ye through the Spirit do mortify the deeds of the body, ye shall live.
Luke 6:46-49 And why call ye me, Lord, Lord, and do not the things which I say?
47 Whosoever cometh to me, and heareth my sayings, and doeth them, I will shew you to whom he is like: heareth Matt 7:24
48 He is like a man which built an house, and digged deep, and laid the foundation on a rock: and when the flood arose, the stream beat vehemently upon that house, and could not shake it: for it was founded upon a rock.
49 But he that heareth, and doeth not, is like a man that without a foundation built an house upon the earth; against which the stream did beat vehemently, and immediately it fell; and the ruin of that house was great.
Philippinas 2:12-13 Wherefore, my beloved, as ye have always obeyed, not as in my presence only, but now much more in my absence, work out your own salvation with fear and trembling.
13 For it is God which worketh in you both to will and to do of his good pleasure.
There are lots more, should I need to go on...or you can visit
this site.