Bucer's influence on Calvin rested on two factors which go hand in hand. First, there was a reciprocal respect which is clearly manifested in their correspondence. This respectful posture toward each other, however, is supported by a second element which relates to the heart of their theology, i.e. the work of the Holy Spirit in causing us to live in communion with Christ. The ‘communio’ that existed between Bucer and Calvin was based on their shared ‘communio cum Christo’.

Joint esteem is clearly evident from the start in the letters which have been preserved from Bucer to Calvin as well as from Calvin to Bucer. Calvin personally felt deeply indebted to his friend from Strasbourg, as he acknowledged to Bullinger in these words: ‘I will not proclaim at this moment the rare and manifold virtues which this man possesses. Let me just say that I would do a great deal of injustice to the church of God if I were to hate or despise him. I will remain silent as to how he made himself serviceable to me personally’. The nature of the friendship between Calvin and Bucer was such that they knew, pointed out and also tolerated each other's weaknesses. This mutual regard and appreciation was deepened in an extraordinary way during Calvin's stay in Strasbourg from 1538 to 1541, where he gave shape to Bucer's ideals while in charge of a small French refugee congregation.