I think for me the most convincing part of Scripture is the Shema:
“Hear, O, Israel, the Lord, Our God - the Lord is One!”
Also Isaiah 45 (BibleHub - NIV):
5 I am the Lord, and there is no other;
apart from me there is no God.
There is only One God, not many. Moses had said that other gods are actually demons.
Deuteronomy 32 KJV:
16 They provoked him to jealousy with strange gods, with abominations provoked they him to anger.
17 They sacrificed unto devils, not to God; to gods whom they knew not, to new gods that came newly up, whom your fathers feared not.
Similarly this is repeated in the NT in 1 Corinthians 10 (KJV):
http://biblehub.com/kjv/1_corinthians/10.htm
14 Wherefore, my dearly beloved, flee from idolatry.
15 I speak as to wise men; judge ye what I say.
16 The cup of blessing which we bless, is it not the communion of the blood of Christ? The bread which we break, is it not the communion of the body of Christ?
17 For we being many are one bread, and one body: for we are all partakers of that one bread.
18 Behold Israel after the flesh: are not they which eat of the sacrifices partakers of the altar?
19 What say I then? that the idol is any thing, or that which is offered in sacrifice to idols is any thing?
20 But I say, that the things which the Gentiles sacrifice, they sacrifice to devils, and not to God: and I would not that ye should have fellowship with devils.
21 Ye cannot drink the cup of the Lord, and the cup of devils: ye cannot be partakers of the Lord’s table, and of the table of devils. 22 Do we provoke the Lord to jealousy? are we stronger than he?
Isaiah 44:8
“Fear ye not, neither be afraid: have not I told thee from that time, and have declared it? ye are even my witnesses. Is there a God beside me? yea, there is no God; I know not any.”
King James Version (KJV)
So God Himself is telling us He is it. There are no other gods in existence but Him.
Also, when speaking about Abraham, Jesus stated this:
John 8:58 - Lexicon in Greek w/ English Translation:
https://www.biblehub.com/lexicon/john/8-58.htm
BibleHub - Strong’s Concordance:
http://biblehub.com/greek/1510.htm
1510 /eimí (“is, am”) – in the present tense, indicative mood – can be time-inclusive (“omnitemporal,” like the Hebrew imperfect tense). Only the context indicates whether the present tense also has “timeless” implications. For example, 1510 (eimí) is aptly used in Christ’s great “I am” (ego eimi . . . ) that also include His eternality (self-existent life) as our life, bread, light," etc. See Jn 7:34, 8:58, etc.
Example: Jn 14:6: “I am (1510 /eimí) the way, the truth and the life.” Here 1510 (eimí) naturally accords with the fact Christ is eternal – maning “I am (was, will be).” The “I am formula (Gk egō eimi)” harks back to God’s only name, “Yahweh” (OT/3068, “the lord”) – meaning “He who always was, is, and will be.” Compare Jn 8:58 with Ex 3:14. See also Rev 4:8 and 2962 /kýrios (“Lord”).
For this the Jews picked up stones to stone Him for blasphemy because He was making Himself out to be God (YHWH, the same of which the Shema is recited), & they understood His meaning - & He did not deny this ever.