Charles Ellicott Commentary Matthew 18:19

"Again I say unto you, that if two of you shall agree on earth as touching anything that they shall ask, it shall be done for them of my Father who is in heaven." — Matthew 18:19 (ASV)

Shall agree on earth — The promise, as before, is dependent on implied conditions. Those who pray must be gathered together in the name of Christ (Matthew 18:20), trusting in His intercession, offering a prayer that is not the utterance of the natural person but the spiritual one, and asking in complete submission to the will of their Father in heaven.

When these conditions are absent, as in the prayer of the sons of Zebedee who asked for what they did not understand (Matthew 20:20), their request might be withheld from them or granted in a very different way than what they had set their hearts on.