We read in Ezekiel 9:
Now the glory of the God of Israel had gone up from the cherub on which it rested to the threshold of the house. And he called to the man clothed in linen, who had the writing case at his waist. And the Lord said to him, “Pass through the city, through Jerusalem, and put a mark on the foreheads of the men who sigh and groan over all the abominations that are committed in it.” And to the others he said in my hearing, “Pass through the city after him, and strike. Your eye shall not spare, and you shall show no pity. Kill old men outright, young men and maidens, little children and women, but touch no one on whom is the mark. And begin at my sanctuary.” So they began with the elders who were before the house.
The word for “mark” is a Hebrew letter, Tav or Taw. It is a kind of cross (the Phoenician version is even closer). As one commentator points out:
The first [command] is addressed to the scribe, to mark with a cross those to be spared. The mark, a kind of tattoo or brand indicating ownership, set aside those who belonged to Yahweh, those who, in the words of the Gospel, hunger and thirst after justice. Christian writers, beginning with Origen and Jerome, not unexpectedly read this as a prefiguring of the cross of Jesus, and their interpretations contributed to the salvific significance of the sign of the cross (Joseph Blenkinsopp, Interpretation commentary on Ezekiel).
Indeed, many centuries after Ezekiel another prophet announced doom on Jerusalem and warned the people to be marked so that they could be delivered from the coming destruction:
Now when they heard this they were cut to the heart, and said to Peter and the rest of the apostles, “Brothers, what shall we do?” And Peter said to them, “Repent and be baptized every one of you in the name of Jesus Christ for the forgiveness of your sins, and you will receive the gift of the Holy Spirit. For the promise is for you and for your children and for all who are far off, everyone whom the Lord our God calls to himself.” And with many other words he bore witness and continued to exhort them, saying, “Save yourselves from this crooked generation.” So those who received his word were baptized, and there were added that day about three thousand souls.
In that case, baptism is the visible means, both in the sight of man and God, for distinguishing those who belong to Jesus from those who are not. It is a pledge that one will will sigh and groan over injustice. As the same Peter will later write:
in the days of Noah, while the ark was being prepared, in which a few, that is, eight persons, were brought safely through water. Baptism, which corresponds to this, now saves you, not as a removal of dirt from the body but as an appeal to God for a good conscience, through the resurrection of Jesus Christ, who has gone into heaven and is at the right hand of God, with angels, authorities, and powers having been subjected to him.