I John [9] If we confess our sins, he is faithful and just to forgive
us our sins, and to cleanse us from all unrighteousness.
There can be no cleansing without forgiveness, and there can be no
forgiveness without confession. It simply is not true that all of our
past, present, and future sins were forgiven when we first became
Christians.
Your syllogism is defective for several reasons. Even you must admit that the propitiation of Christ was for ALL our sins or else there is no forgiveness for ANY of our sins as they were ALL yet future from the cross.
The fact that Christ died for our sins 2000 years ago with ALL of our sins yet future, if He died for any He had to die for ALL. The other problem you have is then ANY SIN - whether omission or commission separates a person from Christ if he happen to die before he could pray.
Second, because of your view of justification by works you fail to distinguish between judicial cleansng and forgiveness for ALL our sins and EXPERIENTIAL application of cleansing.
Third, the Scriptures directy contradict you in the very definition of even what you admit must refer to INITIAL justification:
Rom. 4:7 Saying, Blessed are they whose iniquities are forgiven, and whose sins are covered.
8 Blessed is the man to whom the Lord will not impute sin.
Even what you call "initial" justification takes care of the PRESENT sins when justified (v. 7 "are") as well as the future sins which the Lord "WILL" not impute sin unto.