Philip realized that the Ethiopian was reading from the great Messianic prophecy of Isaiah 53:7,8, and it was natural to use this Scripture to preach Christ to him. All the Old Testament Scriptures relate to Christ in one way or another.

Source:

KJV Defenders Study Bible, by Dr. Henry Morris, Ph.D.
Publisher: Thomas Nelson
Language: English
DEF 10 ISBN 0-529-10444-x
DEF 10-1 ISBN 0-529-10445-8