Baptism justifies and saves

Naaman washed, 7 times, in the Jordan

Baptism does not justify.

Christ justifies through Baptism. Do you, like Naaman, deny that God can work through the water?

2 Kings 5:10 And Elisha sent a messenger unto him, saying, Go and wash in Jordan seven times, and thy flesh shall come again to thee, and thou shalt be clean. 11 But Naaman was wroth, and went away, and said, Behold, I thought, He will surely come out to me, and stand, and call on the name of the Lord his God, and strike his hand over the place, and recover the leper. 12 Are not Abana and Pharpar, rivers of Damascus, better than all the waters of Israel? may I not wash in them, and be clean? So he turned and went away in a rage.

God can even heals through the mud:

John 9:11 He answered and said, A man that is called Jesus made clay, and anointed mine eyes, and said unto me, Go to the pool of Siloam, and wash: and I went and washed, and I received sight.

This is all plainly taught in Scripture.

Christ justifies

Through Baptism.

Baptism is a sign

It is an effective sign which accomplishes what it signifies by the Power of God.

How silly to believe that someone is committing blasphemy by knowing that it is the power of Christ, not water that justifies.

How silly to make that claim. We know that it is Christ working through the Sacrament. That is the Catholic Teaching:

1127 Celebrated worthily in faith, the sacraments confer the grace that they signify. They are efficacious because in them Christ himself is at work: it is he who baptizes, he who acts in his sacraments in order to communicate the grace that each sacrament signifies. The Father always hears the prayer of his Son's Church which, in the epiclesis of each sacrament, expresses her faith in the power of the Spirit. As fire transforms into itself everything it touches, so the Holy Spirit transforms into the divine life whatever is subjected to his power.

But Christ has connected justification and the water by the Word. Therefore, the Sacrament is efficacious:

Ephesians 5:26That he might sanctify and cleanse it with the washing of water by the word,

Certainly we never want to take anything away from baptism,

But you have already in declaring your unbelief that God can use the water to wash away our sins:

Mark 16:16 He that believeth and is baptized shall be saved; but he that believeth not shall be damned.

but we need to realize that it is not baptism that makes one a Christian--it is knowing Christ and being known by Him. 

It is both. Christ has established the Sacraments so that we can know that we are washed of the Spirit and born again children of God:

John 3:2 The same came to Jesus by night, and said unto him, Rabbi, we know that thou art a teacher come from God: for no man can do these miracles that thou doest, except God be with him. 3 Jesus answered and said unto him, Verily, verily, I say unto thee, Except a man be born again, he cannot see the kingdom of God. 4 Nicodemus saith unto him, How can a man be born when he is old? can he enter the second time into his mother's womb, and be born? 5 Jesus answered, Verily, verily, I say unto thee, Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God.

What we must believe to be saved is told to us in Romans 10:9-13,

Romans 10:9 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. 10 For with the heart man believeth unto righteousness; and with the mouth confession is made unto salvation. 11 For the scripture saith, Whosoever believeth on him shall not be ashamed. 12 For there is no difference between the Jew and the Greek: for the same Lord over all is rich unto all that call upon him. 13 For whosoever shall call upon the name of the Lord shall be saved.

Belief doesn't mean "claiming faith". Belief means putting God's words into practice:

Matthew 7:21 Not every one that saith unto me, Lord, Lord, shall enter into the kingdom of heaven; but he that doeth the will of my Father which is in heaven. 22 Many will say to me in that day, Lord, Lord, have we not prophesied in thy name? and in thy name have cast out devils? and in thy name done many wonderful works? 23 And then will I profess unto them, I never knew you: depart from me, ye that work iniquity. 24 Therefore whosoever heareth these sayings of mine, and doeth them, I will liken him unto a wise man, which built his house upon a rock: 25 And the rain descended, and the floods came, and the winds blew, and beat upon that house; and it fell not: for it was founded upon a rock. 26 And every one that heareth these sayings of mine, and doeth them not, shall be likened unto a foolish man, which built his house upon the sand: 27 And the rain descended, and the floods came, and the winds blew, and beat upon that house; and it fell: and great was the fall of it. 28 And it came to pass, when Jesus had ended these sayings, the people were astonished at his doctrine:

The statement that you are justified by taking part in a sign is analogous to the early controversy over circumcision, about which Paul said in Romans 2:25-29,

That's the meaning of Mark 16:16. Any man who is baptized but does not believe in the power of Christ which is poured forth in this sacrament has condemned himself. If a person is baptized who does not believe in Christ, that person has condemned himself.

But let's examine that verse:

Romans 2:25 For circumcision verily profiteth, if thou keep the law: but if thou be a breaker of the law, thy circumcision is made uncircumcision. 26 Therefore if the uncircumcision keep the righteousness of the law, shall not his uncircumcision be counted for circumcision? 27 And shall not uncircumcision which is by nature, if it fulfil the law, judge thee, who by the letter and circumcision dost transgress the law? 28 For he is not a Jew, which is one outwardly; neither is that circumcision, which is outward in the flesh: 29 But he is a Jew, which is one inwardly; and circumcision is that of the heart, in the spirit, and not in the letter; whose praise is not of men, but of God.

Note that there is a spiritual circumcision. What does St. Paul say that is?

Colossians 2:10 And ye are complete in him, which is the head of all principality and power: 11 In whom also ye are circumcised with the circumcision made without hands, in putting off the body of the sins of the flesh by the circumcision of Christ: 12 Buried with him in baptism, wherein also ye are risen with him through the faith of the operation of God, who hath raised him from the dead.

Spiritual circumcision is accomplished in Baptism. As Scripture clearly illustrates.

Likewise, justification is a matter of the heart, by the Spirit.

Absolutely! It is in Baptism that we are washed and regenerated by the Spirit of God:

Titus 3:5 Not by works of righteousness which we have done, but according to his mercy he saved us, by the washing of regeneration, and renewing of the Holy Ghost;