This may surprise you, but there are not very many Bible verses which specifically say that God is eternal. In fact, the words "eternal God" appear only once in the Scriptures. Deuteronomy 33:27 says, "The eternal God is thy refuge, and underneath are the everlasting arms: and He shall thrust out the enemy from before thee; and shall say, 'Destroy them.'" Here, the Hebrew word translated "the eternal" is קֶ֔דֶם (ke·dem), which means "without beginning or end; always existing." The fact that the LORD's name is "I AM" (Ex. 3:14), makes it clear to me that there never was a time when God did not exist.
The only other time the word "eternal" appears in the Old Testament is in Isaiah 60:15, where it would be better translated as "everlasting," since it refers to the city of Jerusalem which obviously had a beginning. There, the Hebrew is עֹולָ֔ם (o·v·lam) which is translated "ever" 267 times, and everlasting 11 times in the Old Testament.
"Eternal" in the New Testament is most often used as an adjective for the word "life." Each of the three verses which describe God as being eternal comes from a different Greek word, and oddly enough, each appears to refer to an individual member of the Trinity. The Father is referred to in Romans 1:20; the Son in 1 Timothy 1:17; and the Holy Spirit in Hebrews 9:14.
Romans 1:20 says, "For the invisible things of Him from the creation of the world are clearly seen, being understood by the things that are made, even His eternal power and Godhead; so that they are without excuse." The Greek is ἀΐδιος (aïdios) which is translated "eternal" one time, and "everlasting" the only other time it is used.
1 Timothy 1:17 says, "Now unto the King eternal, immortal, invisible, the only wise God, be honour and glory for ever and ever. Amen." Here the word is αἰών (aiōn) which is translated "world" 32 times, "age" 2 times, and "eternal" twice, here and in Ephesians 3:11.
Hebrews 9:14 says, "How much more shall the blood of Christ, who through the eternal Spirit offered Himself without spot to God, purge your conscience from dead works to serve the living God?" In this verse, the word translated "eternal" is αἰώνιος (aiōnios). It appears as "eternal" 42 times, "everlasting" 25 times, and "forever" once.
With so few references to God as being eternal, one might wonder why His eternal nature has been emphasized when discussing His attributes. The answer lies in the fact that, unlike His creation, there has never been a time when God did not exist. In fact, God existed before time started, and He will be in existence long after time is no more. To be continued, Lord willing.