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those in the past were baptized also. ( 1 Corinthians 10 )
It is not hard to understand, for the Spirit of the Lord was in full work through Moses.
Without the Spirit of the Lord working in Moses, he could not have done the things He did in the will of God. Paul is saying here that if anybody wants to be continuous about the issue of baptism, look here that our fathers ( predecessor's ) in the faith were baptized through Moses in the cloud ( Holy Spirit ) and the sea ( water ).
Then we can even take other scriptures from the old testament that showed symbolism of baptism, just not the style it is done now, still being used back then;
Exodus 19:10-11
And the LORD said unto Moses, Go unto the people, and sanctify them today and tomorrow, and let them wash their clothes, And be ready against the third day: for the third day the LORD will come down in the sight of all the people upon mount Sinai.
Exodus 29:4
And Aaron and his sons thou shalt bring unto the door of the tabernacle of the congregation, and shalt wash them with water.
Hebrews 9:18-20
Whereupon neither the first testament was dedicated without blood. For when Moses had spoken every precept to all the people according to the law, he took the blood of calves and of goats, with water, and scarlet wool, and hyssop, and sprinkled both the book, and all the people, Saying, This is the blood of the testament which God hath enjoined unto you.
This shows that water has been and always been part of the sanctification process for salvation, it was part of it back then, it was part of it through John the baptist, the Apostles were commanded to still use it, and Jesus nowhere omitted the usage of water in baptism.
Even when His apostles were baptizing around Him, He never once told them to stop or say not needed any more. On the contrary He commanded them to keep doing it.