If you were to ask 100 average Americans on the street if they could tell you about the following words -- divine condescension, providence, atonement, ascetic, salvific, cruciform, prelapsarian, ascension, beseech, prostration, ecumenical, eschaton, anchorite, presbyter, ineffable, incarnate, begotten, sanctified, etc -- I guarantee that the majority would know very little of them. Many people would know absolutely none. Studies show that levels of literacy and especially religious literacy are extremely low.
The point being: Everyone has to learn theological vocabulary to understand the liturgical services, including native speakers of English worshiping in English. The language of Orthodox worship is poetic, theologically rich, and substantially more sophisticated than average speech. That's a real and significant barrier to many people, young and old. Overcoming it requires education, exposure, and effort.
Such is true, in my experience, of most Greeks. Those who are (a) educated and/or (b) spend a lot of time in Church don't have much trouble understanding the Divine Liturgy and common liturgical hymns. As long as you overlook the priestly prayers, the vocabulary and syntax of the fixed parts of the Divine Liturgy are relatively simple.
At least for me, making the switch is about as easy as reading 14th century English. Some people find that hard or even "impossible", but, with a little effort, it's quite easy for a literate speaker of modern English. Consider this famous passage from the Gospel according to St John, translated by John Wycliffe in 1384:
For God louede so the world, that he yaf his `oon bigetun sone, that ech man that bileueth in him perische not, but haue euerlastynge lijf. For God sente not his sone in to the world, that he iuge the world, but that the world be saued bi him. He that bileueth in hym, is not demed; but he that bileueth not, is now demed, for he bileueth not in the name of the `oon bigetun sone of God. And this is the dom, for liyt cam in to the world, and men loueden more derknessis than liyt; for her werkes weren yuele. For ech man that doith yuele, hatith the liyt; and he cometh not to the liyt, that hise werkis be not repreued. But he that doith treuthe, cometh to the liyt, that hise werkis be schewid, that thei ben don in God.