Is the church or Israel the bride of Christ?
In the Old Testament, Israel was depicted as God's bride in several prophecies. But she was declared to be a faithless wife. God declared that He was a husband to Israel (Jeremiah 31:32), but Israel broke the marriage covenant. She became a spiritual adulteress. "So it came to pass, through her casual harlotry, that she defiled the land and committed adultery with stones and trees" (Jeremiah 3:9). In Ezekiel 16, God discusses this at length, calling Israel a degenerate harlot because she won't accept pay for her harlotry, she paid others. ""How degenerate is your heart!" says the Lord GOD, "seeing you do all these things, the deeds of a brazen harlot. You erected your shrine at the head of every road, and built your high place in every street. Yet you were not like a harlot, because you scorned payment. You are an adulterous wife, who takes strangers instead of her husband. Men make payment to all harlots, but you made your payments to all your lovers, and hired them to come to you from all around for your harlotry. You are the opposite of other women in your harlotry, because no one solicited you to be a harlot. In that you gave payment but no payment was given you, therefore you are the opposite."" (Ezekiel 16:30-34). See "Jerusalem as an Unfaithful Woman" for more details.
In the New Testament, the church is depicted as Christ's bride. "Husbands, love your wives, just as Christ also loved the church and gave Himself for her, that He might sanctify and cleanse her with the washing of water by the word, that He might present her to Himself a glorious church, not having spot or wrinkle or any such thing, but that she should be holy and without blemish" (Ephesians 5:25-27). It is the new Jerusalem, not the old, that is presented, "Then I, John, saw the holy city, New Jerusalem, coming down out of heaven from God, prepared as a bride adorned for her husband" (Revelation 21:2). As Paul stated, "For I am jealous for you with godly jealousy. For I have betrothed you to one husband, that I may present you as a chaste virgin to Christ" (II Corinthians 11:2).
The reason for the depiction of God's people as being married to God is that a marriage is formed by a solemn covenant. "Because the LORD has been witness between you and the wife of your youth, with whom you have dealt treacherously; yet she is your companion and your wife by covenant" (Malachi 2:14). Both ancient Israel and the church have entered into a covenant with God. "But now He has obtained a more excellent ministry, inasmuch as He is also Mediator of a better covenant, which was established on better promises. For if that first covenant had been faultless, then no place would have been sought for a second" (Hebrews 8:6-7). Just as a marriage forms an exclusive relationship between a man and a woman, our covenant with God is exclusive. God has the right to protect His relationship (II Corinthians 11:2). He has the right to expect us to be faithful to our commitment.