On the Eucharist
St. Teresa warns us not to receive communion simply out of routine or with a lack of full attention. In her 'Meditations on the Song of Songs' she writes:
'I think that if we were to approach the Most Blessed Sacrament with great faith and love, once would be enough to make us rich. How much richer from approaching so many times as we do. The trouble is we do so out of routine, and it shows.' (M. 3:13)
Living faith recognizes Jesus in the Blessed Sacrament
"After having received the Lord, since you have the Person Himself present, strive to close the eyes of the body and open those of the soul and look into your heart. For I tell you again, and would like to tell you many times that you should acquire the habit of doing this every time - you receive Communion and strive to have such a conscience that you will be allowed to enjoy this blessing frequently. Though He comes disguised, the disguise as I have said, does not prevent Him from being recognized in many ways, in conformity with the desire we have to see Him. And you can desire to see Him so much that He will reveal Himself to you entirely." - St. Teresa of Avila: Way of Perfection 34:12.
Deep Secrets revealed in Communion
"One day, when I had just communicated, I was shown how the Most Sacred Body of Christ is received by His Father within our soul. So now I understand and have these Divine Persons are there, and how pleasing to God is this offering of His Son, since He delights and rejoices in Him, as we may say, here on earth, for it is not His Humanity that is within us in the soul but His Divinity, and it is not for that reason that He is so pleasing and acceptable to Him and that He grants us such favors." - St. Teresa of Avila: Spiritual Testimonies 52 - Vol. 1 pg. 346.