This verse says nothing about being "in Him." The verse you are quoting is in regard to Colossians 1:16-17:
"For in him all things were created: things in heaven and on earth, visible and invisible, whether thrones or powers or rulers or authorities; all things have been created through him and for him. He is before all things, and in him all things hold together" (Col.1:16-17).
Now I have answered you it is your time to answer me about these words of the LORD where He gives Paul a commission to preach to the Gentiles:
"...delivering thee from the people, and from the Gentiles, unto whom I send thee, to open their eyes, that they may turn from darkness to light and from the power of Satan unto God, that they may receive remission of sins and an inheritance among them that are sanctified by faith in me" (Acts 26:17-18).
Here we can see that the LORD sent Paul to the Gentiles so "that they may receive remission of sins."
How could it be possible for those Gentiles to have their sins taken away if you are right and all of men's sins had already been taken away at the Cross?
Nothing is yours until you enter in.
"Therefore if any man be "IN CHRIST", he is a new creation" 2 Corinthians 5:17.
We receive remission of sins when we enter in because we are now "IN CHRIST". God seals us with the Holy Spirit and places us "IN CHRIST".
Did not Christ defeat sin, death and the devil and open the gates of heaven so that... "Whosoever that shall call upon the name of the Lord shall be saved" Romans 10:13.