When, Acts 10:4 says thy prayers and thy alms have come up as a memorial before God, it was referring to Cornelius, his prayers and his offerings or donations had come up before God as a memorial – first of all, what is a memorial?

The Bible defines it as a donation, a gift or being merciful by giving. Apparently Cornelius was a big giver, or at least a faithful giver and his giving along with his prayers had caught God’s attention. Because it had caught God’s attention, God sent an angel to him. If you read the rest of the chapter you will find that because of Cornelius’s giving and prayer, God gave him divine revelation of Jesus, he was baptized in the Holy Ghost and his life was never the same…all because he prayed and gave offerings faithfully.