What matters is confessing to the Lord, "I am a sinner." I don't mean perfunctorily saying it, either. The Lord knows who is sincere and who is insincere.
It is profession of faith and belief in the heart that saves. These are not two separate things but rather two facets of the same thing. Romans 10:9-10 says,
That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead, thou shalt be saved. For with the heart man believeth unto righteousness; and with the mouth confession is made unto salvation.
In the Bible a confession of faith is something on the tip of one's tongue. It is something a believer is ready to do at the drop of a hat. "Oh," but you say, "some are reticent." Fine. I say to you, the reticent give no testimony to men that they are saved. They may indeed be saved, but they give men no evidence of it. One may as well wonder, have they given God evidence of it?
Some will say, "Well, some give evidence of their salvation through godly living." I say to them, godly living includes godly speech. You will notice that we humans speak often. Someone who speaks without ever speaking of Christ, I am not saying they are unsaved, I am saying I have absolutely no testimony from them that they are saved.
But you say what does it matter? It only matters that God knows whether or not one is saved, you say. Well, how then shall believers recognize one another? By going to church every Sunday morning? Oh, what a corruption of the truth of the Word of God! I John 4:2-3 puts it definitively,
Hereby know ye the Spirit of God: every spirit that confesseth that Jesus Christ is come in the flesh is of God; and every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist whereof ye have heart that it should come, and even now already is it in the world.
Any "confession" made on Sunday morning, comprised of a meaningless repetition of words week after week, is meaningless, both to God and to man. Believers may indeed participate in this error; but while men sleep many of the unconverted have crept into the churches unawares. These are making the same "confession" as you. Nominal Christians reading this will respond by taking offense, "Why, these are good people you are talking about!" And thus they prove that the phrase "all men are sinners" is empty verbiage.
Jesus will say of many who prophesied and prayed and confessed in His Name, "I never knew you." By contrast, when I encounter people with the name of Jesus on their tongues, I am very comfortable asking them if they have been saved by Him, and they invariably tell me, "yes, I have!" Thereby we believers are able to recognize one another, and it has nothing to do with empty and rote "church practices."