I shared these helpful memory devices this morning, and perhaps they will be helpful to you as well. Some of the terms theologians use can be somewhat confusing, perhaps just because the words look so big! But the nice thing about the big words, they come from much smaller root words.
In these instances, these are just a different way of stating the big words, to help you remember the importance of their definitions.
Justification = just-as-if I had never sinned. When your sins have been forgiven, you appear to God just as if you had never sinned, so you have been justified by your faith (see Romans 3:22-24).
Atonement = at one-ment with Jesus. Christ said that He was the way, the truth, and the life, and that no one could come into the Father’s presence except through Jesus. So atonement makes at one-ment with Jesus, which allows us to enter into God’s presence (see Romans 3:25).
Sanctification = saint-ification. The Holy Spirit begins a process in our lives where He is forming us into saints. Whereas justification and atonement are one-time events, sanctification is an ongoing process that is conforming us more and more into the image of Jesus (see 2 Thessalonians 2:13).
I hope this helps you understand and apply these terms.