“All the Christians I’ve ever met have been sinners both before and after becoming Christians. If they become mired in sin and DO NOT REPENT they will not be saved. You might have repented ten years ago, but might commit murder and not repent of it. Will you still be saved then?”
The problem is that you are confusing a decision with a new creation. That is why I hate it when someone says, “Ask Jesus to come into your life...”
NO! We die, and are raised a new creation - born again. “”Truly, truly, I say to you, unless one is born again he cannot see the kingdom of God.” Nicodemus said to him, “How can a man be born when he is old? Can he enter a second time into his mother’s womb and be born?” Jesus answered, “Truly, truly, I say to you, unless one is born of water and the Spirit, he cannot enter the kingdom of God. That which is born of the flesh is flesh, and that which is born of the Spirit is spirit.”
If we ‘make a decision for Christ’, we make a decision...but what we need is to be born again. “3Do you not know that all of us who have been baptized into Christ Jesus were baptized into his death? 4We were buried therefore with him by baptism into death, in order that, just as Christ was raised from the dead by the glory of the Father, we too might walk in newness of life.” - Romans 6
“You, however, are not in the flesh but in the Spirit, if in fact the Spirit of God dwells in you. Anyone who does not have the Spirit of Christ does not belong to him. But if Christ is in you, although the body is dead because of sin, the Spirit is life because of righteousness...For if you live according to the flesh you will die, but if by the Spirit you put to death the deeds of the body, you will live. For all who are led by the Spirit of God are sons of God. For you did not receive the spirit of slavery to fall back into fear, but you have received the Spirit of adoption as sons, by whom we cry, “Abba! Father!” The Spirit himself bears witness with our spirit that we are children of God...” Romans 8
If you have the Holy Spirit in you, you WILL repent. Not maybe, or might - you WILL, for it is God Himself convicting you. Jesus Himself said He would see us through to the end.
The assurance of the believer isn’t required for salvation, but it will follow with time. “...but you have received the Spirit of adoption as sons, by whom we cry, “Abba! Father!” 16 The Spirit himself bears witness with our spirit that we are children of God”
Is it possible to deceive ourselves? Yes. There are people who deceive themselves into believing they are prophets - but that doesn’t mean prophets don’t exist. There are people who deceive themselves into thinking they are flowers - but that doesn’t mean flowers do not exist. There are people who deceive themselves about being children of God - but that doesn’t mean no one has been born again.
“If I were to put several supposedly Bible believing Protestants in a room and ask them to discuss baptism they would come to different conclusions.”
When I’ve done it, the answer isn’t quite as contrary as you indicate. I can’t think of any Protestant I’ve met who objected to Believer’s Baptism. Some - typically those that have been aligned with a government power at some time - permit infant baptism. As imperfect men, ALL of us have to fight against interpretations tainted with what we wish it said, versus what Scripture actually says. However, on matters of importance, it simply isn’t all that difficult.
You wrote:
“The problem is that you are confusing a decision with a new creation.”
Nope. I am not confusing anything here. I rarely confuse things. You are the one confusing things. Even a new creation must make a decision. If decisions did not have to be made then Christians would NEVER ever be tempted to sin Are you ever tempted to sin? Then, gee, you must not be a new creation then, right? No, of course you are, but we must still choose Christ every day.
“If we ‘make a decision for Christ’, we make a decision...but what we need is to be born again.”
Those born again are still tempted - hence DECISIONS are made. We can choose well or poorly, but we still decide. God gives us the grace we need to resist sin, but we still must choose to resist that temptation.
The rest of wha you said in no way overturned that irrefutable logic. It is also entirely scriptural. Hence, the sacred writers - and God spaking through them - repeatedly warn us to avoid sin.