Is mental prayer necessary for salvation? Might sound a bit extreme. The thought may spur feelings similar to hearing your junior high teacher deliver a pop quiz which you were not prepared for. I remember this to have been the case–only to find out after the quiz that the answers were a part of my assigned reading for the past 5 days.
In order to address this question, we should know WHAT mental prayer is, and make an important distinction to more clearly address the question.
What mental prayer is: According to Saint Teresa, mental prayer is a heart to heart conversation with Christ in our soul. Period. It requires self knowledge of who we are, inspiring a right reverence for Our Blessed Lord–but also a deep piety and trust! It is, really, the essence of the spiritual life: it is living the life of grace given through baptism and exalted toward Eucharistic union! That is, it is aiming to abide with God (Who is in our soul), foreshadowing the beatific vision.
Distinguishing the formal exercise from its spirit: This activity uses both our intellect to reflect on truths and our will to love. The latter is the essence of prayer, but the former is necessary in the beginning of prayer to inspire light and devotion for the will to grab on to with love. Thus, mental prayer is known for most as a time of meditation: reflection on eternal truths of our faith leading to intimacy with Christ–speaking to Him with our own words, listening to Him, sharing of affection, and even simply being in His presence silently. It is like learning how to drive: in the beginning, you have to think of all the steps (put in drive, push the pedal, etc.); but once experienced, you simply “drive”. Prayer becomes like this, but it takes time to instill the habits. That said, the formal activity of mental prayer in which one goes through steps should be differentiated from the natural inclination we have as Christians to consider eternal truths and love God; the liturgy should inspire this intuitively. If you are doing this latter, they are technically doing mental prayer without knowing it! This is the morally necessary element of mental prayer, in so far as it orients us toward our final end and helps bring light to our faults.