Inner Assurance

And because you are children, God has sent the Spirit of his Son into our hearts.  Galatians 6:4a

 We must not doubt that the Holy Spirit dwells in us. We must be sure and acknowledge that we are a "temple of the Holy Spirit" (1 Cor. 6:19). For if someone experiences love toward the Word, and if he enjoys hearing, speaking, thinking and writing about Christ, he should know that this is not a work of human will or reason but a gift of the Holy Spirit.

And therefore we should believe that whatever we say, do, or think is pleasing to God, not on our account but on account of Christ. We are most certain that Christ is pleasing to God and that he is holy. To the extent that Christ is pleasing to God and that we cling to Him, we too, are pleasing to God and holy. Although sin still clings to mour flesh and we still fall every day, still grace is more abundant and more powerful than sin.

This inner assurance that we are in a state of grace and have the Holy Spirit is accompanied by the external signs I have mentioned: to enjoy hearing about Christ; to teach, give thanks, praise, and confess Him; to do one's duty according to one's calling; to help the needy and comfort the sorrowful. By these signs we are assured and confirmed that we are in a state of grace.
Lectures On Galatians, Luther's Works 26:376,378-79