Imam Sadiq (عليه السلام) in a lengthy narration:

ثم انى أخبرك ان الدين واصل الدين هو رجل وذلك الرجل هو اليقين وهو الايمان وهو امام أمته وأهل زمانه فمن عرف عرف الله ومن أنكره أنكر الله ودينه ومن جهله جهل الله ودينه وحدوده وشرايعه بغير ذلك الامام كذلك جرى بان معرفة الرجال دين الله

"Then I hereby inform you that the Religion and the foundation of Religion,  is a man, and that man itself is certainty (yaqeen), and He is faith, and He is the Imam (asws) of His community and the Imam of the people of His time. So the one who recognizes Him has recognized Allah (swt), and the one who denies Him has denied Allah (swt), and His religion, and the one who is ignorant of Him is ignorant of Allah (swt) and His Religion, and His Limits, and His laws. Hence it happens to be that the Recognition of that man is the religion of Allah (swt)" 

[Source: Basair al-Darajaat, Pg, 539]

The topmost matter in Religion?

عليُّ بْنُ إِبْرَاهِيمَ عَنْ أَبِيهِ عَنْ حَمَّادِ بْنِ عِيسَى عَنْ حَرِيزٍ عَنْ زُرَارَةَ عَنْ ابي جعفر (عَلَيْهِ السَّلام) قَالَ ذِرْوَةُ الامْرِ وَسَنَامُهُ وَمِفْتَاحُهُ وَبَابُ الاشْيَاءِ وَرِضَا الرَّحْمَنِ تَبَارَكَ وَتَعَالَى الطَّاعَةُ لِلامَامِ بَعْدَ مَعْرِفَتِهِ ثُمَّ قَالَ إِنَّ الله تَبَارَكَ وَتَعَالَى يَقُولُ مَنْ يُطِعِ الرَّسُولَ فَقَدْ أَطاعَ الله وَمَنْ تَوَلَّى فَما أَرْسَلْناكَ عَلَيْهِمْ حَفِيظاً

Narrated that Abu Jafar Imam Baqir (عليه السلام) said,

"The topmost matter (in religion), its peak, its key, the gateway to all affairs and the pleasure of the Most Beneficent, the Most Holy, the Most High, is obedience to the Imam (ع) after recognizing the Imam." 

The Imam (ع) then said, “Allah (تبارك و تعالى) has said, ‘One who obeys the Messenger has certainly obeyed Allah, and whoever turns away, then we have not sent you as a watcher over them." (4:80)

[Source: al-Kafi, Vol. 1, Pg. 185]

Connecting with Imam al-Mahdi (ع) .. read the below 

1) A Dua (supplication) to be recited in the period of Ghaybah as taught by Imam Mahdi (ع), recorded by Shaikh Sadooq in his book Kamal al-Deen:

http://duas.org/pdfs/Allahummaarrifni.pdf 

2) A Salawaat taught by Imam Mahdi (ع) recorded by Shaikh Toosi in his book Misbah al-Mutahajjid:

https://www.duas.org/imamezaman/zarrab_isfahani.htm

3) Ziyarat Aal-e-Yaseen as taught by Imam Mahdi (ع), recorded by Shaikh Tabarisi in his book al-Ehtejaaj:

http://www.duas.org/pdfs/ZiaratAleyasin.pdf

4) Dua taught by Imam Ridha (ع), recorded by Shaikh Tusi in his boom Misbah al-Mutahajjid:

http://www.duas.org/imamezaman/dua_sardaab.htm