And when Jesus was baptised, immediately he went up from the water, and behold, the heavens were opened to him, and he saw the Spirit of God descending like a dove and coming to rest on him; and behold, a voice from heaven said, “This is my beloved Son, with whom I am well pleased.” (Mt 3:16-17)
When the New Testament writers use the word "behold!" they do so to get our attention. They do so because what follows is of special significance and importance. The word "behold" itself in the English is not ideal, because it is not widely used today and so can make a passage seem dated or religious, and that certainly is not the intention. But it is hard to think of a better alternative, especially considering the literal translation is the imperative for look.
Here's what the translators of the ESV had to say about it:
The word “behold,” usually has been retained as the most common translation for the Hebrew word hinneh and the Greek word idou. Both of these words mean something like “Pay careful attention to what follows! This is important!” Other than the word “behold,” there is no single word in English that fits well in most contexts. Although “Look!” and “See!” and “Listen!” would be workable in some contexts, in many others these words lack sufficient weight and dignity. ~ Preface to the English Standard Version
There is no single English word that fits. One phrase I thought of was, "Mark my words!" In a language that originally lacked punctuation, it could also bee seen as a literary device similar to the exclamation mark or bold italics.
At the end of Matthew chapter 3, the account of Jesus baptism, we get two such "behold!" moments in rapid succession. The first when the heavens are opened to Jesus, and Holy Spirit descends upon him in manifest form like a dove, and the second when following close behind the Spirit comes the voice from heaven declaring, "This is my Son, the beloved, upon whom is my favour."
Father, son and Spirit are involved significantly from the very outset of Jesus ministry. Behold: Heaven is opened. The Spirit is manifested. There is a sound from heaven.
In the book of Acts we see that Jesus continues to work the way he began...
And suddenly there came from heaven a sound like a mighty rushing wind, and it filled the entire house where they were sitting. And divided tongues as of fire appeared to them and rested on each one of them. And they were all filled with the Holy Spirit and began to speak in other tongues as the Spirit gave them utterance. (Acts 2:2)