The Jewish law required you to be purified before you came into the presence of God. A person needs to be in a status of ritual purity in order to enter the Temple area. If the person has been affected by a source of impurity they need to undergo a process of ritual purification before they can again enter the Temple or be put to use for work related to the Temple.
The practice of tevilah, the immersion of the entire body in water for the purpose of removing ritual impurity was a practice related to baptism.
When you are baptized it symbolizes your death to yourself and buried with Christ and raised up a new man cleansed from your impurities.
Romans 6:4 Therefore we are buried with him by baptism into death: that like as Christ was raised up from the dead by the glory of the Father, even so, we also should walk in newness of life.
In the New Testament John the Baptist called for repentance. People came to confess their sins and was baptized fully immersed by John. When Jesus was baptized he came up out of the water and the Spirit of God came upon him.
Matthew 3:16 And Jesus, when he was baptized, went up straightway out of the water: and, lo, the heavens were opened unto him, and he saw the Spirit of God descending like a dove, and lighting upon him:In Acts, Peter said to be baptized and you shall receive the gift of the Holy Spirit.
Acts 2:38 Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost.
When you are baptized you are baptized into Jesus death.
Romans 6:3 Know ye not, that so many of us as were baptized into Jesus Christ were baptized into his death?
Raised up a new person and have put on Christ.
Galatians 3:27 For ye are all the children of God by faith in Christ Jesus. For as many of you as have been baptized into Christ have put on Christ.
So when we are baptized, we come into the presence of God through the Spirit of Truth.
So baptism is the start of our new life in Christ, burying the old man and rising a new man in Christ.
1 Peter 3:21 We then go on daily cleansing our inner man by obeying the truth.
1 Peter 1:22 Seeing ye have purified your souls in obeying the truth through the Spirit unto unfeigned love of the brethren, love one another with a pure heart fervently: And we are born again by the word of God, born once in the flesh and a second time in the spirit.
1 Peter 1:23 Being born again, not of corruptible seed, but of incorruptible, by the word of God, which liveth and abideth forever.
We grow as newborn babies with the milk of the word until we become adults in God.
1 Peter 2:2 As newborn babes, desire the sincere milk of the word, that ye may grow thereby:
1 John 3:2 Beloved, now are we the sons of God, and it doth not yet appear what we shall be: but we know that, when he shall appear, we shall be like him; for we shall see him as he is.