We see here the apostles were astonished because God gave the gentiles the Holy Ghost.We see in the scripture that tongues is a sign to the unbelievers.Who was the unbelievers here ? The ones who didn’t believe God gave the Holy Ghost to the gentiles,they were unbelievers because they didn’t believe God was giving the Holy Ghost to the gentiles.So tongues was a sign to the apostles that God gave the Holy Ghost to the gentiles.You see the apostles were not unbelievers in that they didn’t believe in God but they were unbelievers because they didn’t believe God was giving the Holy Ghost to the gentiles.So many times people speak in tongues because they don’t believe they are filled unless they do speak in tongues.
Acts10: While Peter yet spake these words, the Holy Ghost fell on all them which heard the word.
 And they of the circumcision which believed were astonished, as many as came with Peter, because that on the Gentiles also was poured out the gift of the Holy Ghost.
 For they heard them speak with tongues, and magnify God. Then answered Peter,
 Can any man forbid water, that these should not be baptized, which have received the Holy Ghost as well as we?
 And he commanded them to be baptized in the name of the Lord. Then prayed they him to tarry certain days
1 Cor.14:22] Wherefore tongues are for a sign, not to them that believe, but to them that believe not: but prophesying serveth not for them that believe not, but for them which believe