Do you believe this scripture to be true:
Hebrews 5:9
And being made perfect, He became the author of eternal salvation unto all them that obey Him;
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It says He is the author of salvation to those who obey Him, and what we obey is what He taught and commanded in the gospel books of Matthew, Mark, Luke, and John that all the Apostle expounded on.
Nowhere does the bible say future sins are automatically forgiven, and in order for that to be true then you would have to do away with Romans 3:25, 2 Peter 1:9, and 1 John 1:7-2:2.
The part you are referring to is He was the only sacrifice needed for sins once and for all, but He still told us how we are to receive that remission through His blood. Repentance, baptism, confession, forgiving others, and walking in love !!!