When you come to Christ you are immediately justified in the sense I you died the next day you die in a saved state because you have accepted Jesus as your saviour
However, the justification Paul is speaking of in gal2:15-18 is seeking to be justified in respecting to crossing over from being slaves of sin when we initially come to Christ into slaves of righteousness leading to holiness(rom6:17&18)
Though a christian I never completely sin free on this earth, there are sins that by their very nature will utterfly consume us if not dealt with and stop us moving forward to the place of being led into holiness. These sins would dominate us and leave us slaves to ynfodliness. EG alcoholism, serial affairs, heavy gambling,. These things need to ve dealt with in the crossing over stage Paul speaks of. Some sin we can imnediatly cease ourselves for it has not tied itself so tightly around us, but sin we are a slave to, that controls us we cannot of ourselves deal with. We must look to Christ and trust him, firstly that we are saved by a sole righteousness of him dying for our sins, and secondly to deliver us from the sin we are a slave to by the holy spirits power.
But Paul tells us Thi crossing over period is NOT instantaneous. Example. An alcoholic of thirty years becomes a christian. They stand by a righteousness of faith in Christ while they earnestly trust him to deal with the alcohol problem. According to Paul words they are found to be a sinner during this tine. But they gate the sin for they gave been born again, the last thing they want is for it to Sta there, but rather than them try to dwfeat it themselves to be justified before God they must look to Christ and trust him to dwal with it. From personal experience I know it I hard to look to Jesus during this crossing over period and trust him you are still saved despite the evident sin in your life. Many give up and walk away, believing they cannot be good enough for God. But if you persevere with standing in your one and only righteousness before God you will see such sins as mentioned defeated. You will then gave crossed over to being a slave of righteousness leading to holiness.
Then the sanctification begins. Living an ever more holy life for God, free of the sins that would stop you doing this. You are a continual work in progress your whole life ad you continue to stand in Christ alone as your righteousness and relying on the continual sanctification of the holy spirit
I am using a Mobil today. It is not was to explain as I would like to, it seems i am rushing it. If you would Lim to read of it fully explained please go to Assemblies of God/pentecostal forum. One of the threads is entitled Grace. Chapters 4,5&6 deal specifically with this issue
God bless and