Per the Nicene Creed:
He became incarnate from the virgin, Mary, and was made man
Per the Holy Bible:
All right then, the Lord himself will give you the sign. Look! The virgin will conceive a child! She will give birth to a son and will call him Immanuel (which means ‘God is with us’).
The angel replied, “The Holy Spirit will come upon you, and the power of the Most High will overshadow you. So the baby to be born will be holy, and he will be called the Son of God. What’s more, your relative Elizabeth has become pregnant in her old age! People used to say she was barren, but she has conceived a son and is now in her sixth month. For the word of God will never fail.”
Mary responded, “I am the Lord’s servant. May everything you have said about me come true.” And then the angel left her.
A few days later Mary hurried to the hill country of Judea, to the town where Zechariah lived. She entered the house and greeted Elizabeth. At the sound of Mary’s greeting, Elizabeth’s child leaped within her, and Elizabeth was filled with the Holy Spirit.
Elizabeth gave a glad cry and exclaimed to Mary, “God has blessed you above all women, and your child is blessed. Why am I so honored, that the mother of my Lord should visit me? When I heard your greeting, the baby in my womb jumped for joy. You are blessed because you believed that the Lord would do what he said.”
Mary responded, “Oh, how my soul praises the Lord."
* * * * * * *
Just so we don't turn "incarnate" into some mystical, purely-religious term, let's remind ourselves that it has a universally understood definition: "embodied in flesh; given a bodily, especially a human, form."
Truth is...While occupying Planet Earth, Jesus was not a visible spirit, magically hovering six inches off the ground and never quite "one of us." When people refer to Jesus as "God in the flesh," that is exactly what they mean. Jesus had a human body, just like you and me.