A matter of simple logic.
But do you believe in Jesus? Do you really and sincerely believe in Jesus? Give me a break. Where do you see all those adjectives in the New Testament? I'm not even going to get into the question of whether you believe in Jesus enough because there is no "enough." Any faith in Jesus at all saves. And you're no more justified how well you believe than by how well you do anything else.
Salvation isn't about you. It's about Jesus. It isn't about what do. It's about what He has done.
But the fact remains that if we do what OCD tells us to do- to look inside ourselves to discover whether or not we believe- we will never find assurance there. The strongest faith is mixed with doubt. That's why Jesus gave us an external, objective, verifiable historical fact to look to instead.
If you put your trust in the promise Jesus made in baptism, you put your promise in Jesus. And your baptism is a historical fact, a datable event. Sure, people can be baptized and fall away, or even be baptized without believing. There is no magic here. But people who don't trust in Jesus don't look for reassurance to the promise Jesus makes in baptism!
That is where certainly resides because that's where Jesus has placed it (by the way, while it's true that in classical Greek the word "baptize" means "immerse," the New Testament was written in Koine Greek- a dialect further removed from classical Greek than American English is from Shakespearian English- and that in Koine it simply means "wash." Matthew even uses it to describe the ritual washing of tables and couches!).
If you doubt your salvation, ask yourself whether you're baptized! Sure, you can fall away from the faith by willful and unrepented sin. But people who do that don't ask the question! And even if you fear that you're in danger of that, remember what baptism is all about:
What is Baptism?--Answer: Baptism is not simple water only, but it is the water comprehended in God's command and connected with God's Word.
Which is that word of God?--Answer: Christ, our Lord, says in the last chapter of Matthew: Go ye into all the world and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost.
What does Baptism give or profit?--Answer: It works forgiveness of sins, delivers from death and the devil, and gives eternal salvation to all who believe this, as the words and promises of God declare.
Which are such words and promises of God? Answer: Christ, our Lord, says in the last chapter of Mark: He that believeth and is baptized shall be saved, but he that believeth not shall be damned.
How can water do such great things?--Answer: It is not the water indeed that does them, but the word of God which is in and with the water, and faith, which trusts such word of God in the water. For without the word of God the water is simple water and no baptism. But with the word of God it is a baptism, that is, a gracious water of life and a washing of regeneration in the Holy Ghost, as St. Paul says, Titus, chapter three: By the washing of regeneration and renewing of the Holy Ghost, which He shed on us abundantly through Jesus Christ, our Savior, that, being justified by His grace, we should be made heirs according to the hope of eternal life. This is a faithful saying.
What does such baptizing with water signify?--Answer: It signifies that the old Adam in us should, by daily contrition and repentance, be drowned and die with all sins and evil lusts, and, again, a new man daily come forth and arise; who shall live before God in righteousness and purity forever.
Where is this written?--Answer. St. Paul says in Romans, chapter 6: We are buried with Christ by Baptism into death, that, like as He was raised up from the dead by the glory of the Father, even so, we also should walk in newness of life.
To claim the promise of baptism is to claim the promise of new life, a new start, a fresh beginning, and a new identity as a child of God, perfect in His sight with the perfection of Jesus Himself. Your weakness doesn't matter here. The strength of your resolution to better doesn't matter; if you want what baptism offers, you desire to do better, and that's all that counts. To trust the promise is to want what the promise promises.
The weakness of your faith doesn't matter; all that matters is that you cling to the promise. The unbelief that always is mixed together with faith doesn't matter. It's not your response to the promise that matters, but the promise itself: a promise not dependent on something which may or may not be present within you, but to an objective, historical fact in time and space, a personalized promise that doesn't come and go with the wavering of your fickle and fallible heart but is always there, as certain as only a promise from God Himself can be,