Baptism by immersion in water was so essential that Christ, who did no sin, was baptized to show us the way. He thus taught that a man must born of water and the spirit to enter the kingdom of God. Ignore it at your your own peril but it is essential to receive baptism for the remission of sins.
Jesus promised the man on the cross the following:
Luke 23:43
43 And Jesus said unto him, Verily I say unto thee, To day shalt thou be with me in paradise.
The key word in this verse is "paradise". Paradise is not the same as "heaven". On the very day that Jesus died, he did not ascend to heaven. We know this because on the third day after Jesus had resurrected, he told Mary the following:
John 20:16-17
16 Jesus saith unto her, Mary. She turned herself, and saith unto him, Rabboni; which is to say, Master.
17 Jesus saith unto her, Touch me not; for I am not yet ascended to my Father: but go to my brethren, and say unto them, I ascend unto my Father, and your Father; and to my God, and your God.
Here we learn that after Jesus resurrected and appeared before Mary on the 3rd day that he had not yet ascended to heaven where his Father was. The thief on the cross could not have been with Jesus in heaven. So the question is, where was Jesus on the day he died when he promised the man on the cross that today you will be with me in paradise? Between the time Jesus died and the time he was resurrected, Jesus went and preach to the spirits of those that have died.
1 Peter 3:18-20
18 For Christ also hath once suffered for sins, the just for the unjust, that he might bring us to God, being put to death in the flesh, but quickened by the Spirit:
19 By which also he went and preached unto the spirits in prison;
20 Which sometime were disobedient, when once the longsuffering of God waited in the days of Noah, while the ark was a preparing, wherein few, that is, eight souls were saved by water.
1 Peter 4:5-6
5 Who shall give account to him that is ready to judge the quick and the dead.
6 For for this cause was the gospel preached also to them that are dead, that they might be judged according to men in the flesh, but live according to God in the spirit.
Jesus went among those who have died who were spirits and set up an organization for them to hear the gospel. Those in the spirit world who have received the gospel are in paradise and those who have not are in spirit prison. The thief on the cross who was to be with Jesus in paradise on that very day was with Jesus among the spirits of the dead who accepted the gospel.
Thus paradise is that place in the world of spirits where those who have received the gospel await the day of resurrection and judgement.