We have beaten this point over and over, but it keeps coming back. Jack Kelley does a nice treatment of this here:
Q. I recently read an article that claimed the Bible offers no proof that our future sins are forgiven. It says this is just an assumption that Eternal Security advocates have made. It says if that was the case we wouldn’t need Jesus to be our advocate (lawyer), and that Romans 3:24-25 proves that only our past sins were forgiven. I’ve thought of Colossians 2:13-14 “He forgave us all our sins …” as a rebuttal. Are there others?
A. I read the excerpts you sent me and find them to be full of theological holes. In the first place there is Scriptural proof that all our sins are forgiven. Along with Colossians 2:13-14 that you mentioned there are also Ephesians 1:13-14 and 2 Cor. 1:21-22 which say our inheritance was assured from the moment we believed, and that God has taken ownership of us and put His spirit in our hearts as a deposit to guarantee this. How could our inheritance be guaranteed from the moment of belief unless all the sins of our life had been forgiven?
The critical word in these passages is not seal, as the article suggests, it’s guarantee. The Greek word translated guarantee means earnest. It’s a legal term describing a deposit that obligates the purchaser to complete the transaction. When God set His mark of ownership on us and put His Spirit in our hearts (2 Cor. 1:21-22), He was giving us His word that He would redeem us. There is nothing we or anyone else in Heaven or on Earth can do that will cause Him to break His word (John 6:38-40, John 10:27-30, Romans 8:38-39).
Similarly, Hebrews 10:14 says by His one sacrifice the Lord has made us perfect forever, and 2 Cor. 5:17,21 says if we’re in Christ we’re a new creation having the righteousness of God. (The verbs in these two verses are all in the past perfect tense. That means they’ve already happened and they are complete.)
The argument that we wouldn’t need an advocate (lawyer) if our violations were already paid for is also flawed. Common sense tells you that even if you’ve been unjustly hauled into court, you’d better have a lawyer. Remember, Satan is our accuser and he’s an unjust one at that.
And Romans 3:24-25 is being taken out of context. The past sins Paul was talking about are the ones people had committed before the cross. Because of their faith in the promised Redeemer God permitted the sacrificial system to set their sins aside until Jesus came to pay for them in full.
What you won’t find in the New Testament is any indication that having obligated Himself to save you, God will subsequently break His word and revoke His salvation. Once we are saved we are always saved.