The simplest way to understand the meaning of a word is to study its usage in as many texts as possible. Fortunately we are saved the very hard work that such a study would involve because a 19th Century scholar, T.J.Conant, in his book, The meaning and use of Baptizein (Wakeman Classic Reprints. ISBN 1-870855-31-0 ), has given every single usage of Baptizo in Classical or Koine Greek, and as we study the word, we can see that it always and invariably means, ‘To dip’, ‘submerge’ or ‘to immerse.’ Sometimes the usage is figurative, as when people are said to be ‘overwhelmed’ by grief, drink, debt or desire. Very occasionally, ‘sprinkle’ is a possible alternative translation, but never once is it required by the context.
It may be helpful to see how some of the Church fathers write about baptism. This is not out of respect for their theological understanding, which was variable to say the least. Rather, we look to them to see what Greek-speakers in the early centuries A.D. understood baptizo to mean, and taught their congregations.
1. Cyril of Jerusalem, Instruction III, on Baptism XII. ‘For as Jesus assuming the sins of the world died, that having slain sin He might raise you up to righteousness; so also you, going down into the water, and in a manner buried in the waters as He in the rock, are raised again, walking in newness of life.’
2. John Chrysostom. Comment on 1Cor. Discourse XL. I. ‘For to be baptized, and to sink down, then to emerge, is a symbol of the descent into the underworld, and of the descent from there. Therefore Paul calls baptism, the burial, saying, “we were buried therefore, with Him by the baptism into death.”‘
3. Athanasius. Discourse on the Holy Passover, 5. ‘In these benefits you were baptized, O newly-enlightened; the initiation into the grace……has become to you an earnest of resurrection; you have the baptism as a surety of the abode in heaven. You imitated, in the sinking down, the burial of the Master; but you rose again from there, before works, witnessing the works of the resurrection.’
4. Gregory of Nazianus. Discourse XL, on the holy Baptism. ‘Let us therefore be buried with Christ by the baptism, that we mayalso rise with Him. Let us go down with Him, that we may also be exalted with Him; let us come up with Him, that we may also be glorified with Him.’
In every single example that I can find (and there are plenty of others), the Church Fathers understand Baptizo to mean immersion. They wrote in Greek; they ought to know what a Greek word means.