Pouring and Sprinkling

The history of pouring and sprinkling is debated, some teach it came from idolatry where there was an infant initiation. This is likely why pouring became accepted. Those coming from idolatry were simply used to it.

On the other hand there are other explanations coming from Gnosticism.

Irenaeus in refuting Marcus and his gnostics wrote,

4. But there are some of them who assert that it is superfluous to bring persons to the water, but mixing oil and water together, they place this mixture on the heads of those who are to be initiated, with the use of some such expressions as we have already mentioned. And this they maintain to be the redemption.

and then he mentions the same practice at death.

 Others still there are who continue to redeem persons even up to the moment of death, by placing on their heads oil and water, or the pre-mentioned ointment with water, using at the same time the above-named invocations, that the persons referred to may become incapable of being seized or seen by the principalities and powers,

This is very similar to what James wrote, in praying for those who were sick or overcome by sin.

Jas 5:13  Is any among you afflicted? let him pray. Is any merry? let him sing psalms.

Jas 5:14  Is any sick among you? let him call for the elders of the church; and let them pray over him, anointing him with oil in the name of the Lord:

Jas 5:15  And the prayer of faith shall save the sick, and the Lord shall raise him up; and if he have committed sins, they shall be forgiven him.

It seems the gnostics confused prayer for the sick as described by James with baptism.  This is likely one of the major reasons pouring and sprinkling came into the church. As  Gnosticism spread it slowly mixed into the church.

Irenaeus wrote about AD 160, at that time he did not accept pouring for baptism as applied by Marcus.