There is a distinction between Justification and Sanctification. Justification is solely the work of God. We thus have the righteousness of Jesus and so God declares that we are righteous through our faith in Christ and Christ alone. (Rom. 4:3; 5:1,9; Gal. 2:16; 3:11).
Sanctification is God working through us to produce a Christ-like character even though we have been justificed (Phil 2:13). We do not have instant sanctification because it is a process where we have to actively submit to the working of the Holy Spirit in our life.
We have to resist sins and seek holiness as we grow in the Lord. This is not legalism because Sanctification does not take away Justification. That is to say even though we may live imperfect life, we are still justified.
To confuse Sanctification with the filthy works of man and legalism is dangerous. The Bible is clear that we are to live holy lives and to avoid sin (Col. 1:5-11). Yes, position-wise, we are eternally saved and justified. We cannot earn our salvation. However, that does not mean that we continue to live in the sin from which we were redeemed.
Yes, we are continually affected by the temptations to sin and the battle between the redeemed of the Lord and sin will continue (Rom. 7:14-20) until the Second Coming. Only then, that we will be delivered from our bodies of death (Rom. 7:24). To claim that since we have been forgiven by the complete work of Christ and therefore we do not need to avoid sins and live a holy life is to trample the blood of Christ underfoot (Heb. 10:29).
Rev Albert Kang