According to the Cambridge Dictionary:
something that you say or do that shows you do not respect God or a religion
According to Paul:
Indeed you are called a Jew, and rest on the law, and make your boast in God, and know His will, and approve the things that are excellent, being instructed out of the law, and are confident that you yourself are a guide to the blind, a light to those who are in darkness, an instructor of the foolish, a teacher of babes, having the form of knowledge and truth in the law.
You, therefore, who teach another, do you not teach yourself? You who preach that a man should not steal, do you steal? You who say, “Do not commit adultery,” do you commit adultery? You who abhor idols, ado you rob temples? You who make your boast in the law, do you dishonor God through breaking the law?
For “the name of God is blasphemed among the Gentiles because of you,” as it is written.
-- Romans 2:17-24 (NKJV)
It is considered a very serious trespassing if you dishonour God with your hypocritical acts.
And Nathan said to David, “The LORD also has put away your sin; you shall not die. However, because by this deed you have given great occasion to the enemies of the LORD to blaspheme, the child also who is born to you shall surely die.” -- 2 Samuel 12:13-14 (NKJV)
God's name is more important than the comfort of a nation:
Moreover the word of the LORD came to me, saying: “Son of man, when the house of Israel dwelt in their own land, they defiled it by their own ways and deeds; to Me their way was like the uncleanness of a woman in her customary impurity. Therefore I poured out My fury on them for the blood they had shed on the land, and for their idols with which they had defiled it. So I scattered them among the nations, and they were dispersed throughout the countries; I judged them according to their ways and their deeds. When they came to the nations, wherever they went, they profaned My holy name — when they said of them, ‘These are the people of the Lord, and yet they have gone out of His land.’ But I had concern for My holy name, which the house of Israel had profaned among the nations wherever they went.
“Therefore say to the house of Israel, ‘Thus says the Lord GOD: “I do not do this for your sake, O house of Israel, but for My holy name’s sake, which you have profaned among the nations wherever you went. And I will sanctify My great name, which has been profaned among the nations, which you have profaned in their midst; and the nations shall know that I am the LORD,” says the Lord GOD, “when I am hallowed in you before their eyes.
-- Ezekiel 36:16-23 (NKJV)
This is because the third commandment given to the Israelites stated:
You shall not take the name of the LORD your God in vain, for the LORD will not hold him guiltless who takes His name in vain. -- Exodus 20:7 (NKJV)